diff --git a/backend/app/services/onetime/backfill_dividend_investment_amounts.rb b/backend/app/services/onetime/backfill_dividend_investment_amounts.rb
new file mode 100644
index 0000000000..1f616e1806
--- /dev/null
+++ b/backend/app/services/onetime/backfill_dividend_investment_amounts.rb
@@ -0,0 +1,111 @@
+# frozen_string_literal: true
+
+# Onetime script to backfill investment_amount_cents on dividends
+#
+# Dividend rounds 9 and 12 are skipped
+# because their investors were created via CSV import without corresponding share_holdings
+# or convertible_securities records, the original CSV is needed for those.
+#
+# Usage:
+#   Onetime::BackfillDividendInvestmentAmounts.perform(dry_run: true)   # Preview only
+#   Onetime::BackfillDividendInvestmentAmounts.perform(dry_run: false)  # Actually update
+class Onetime::BackfillDividendInvestmentAmounts
+  SKIPPED_DIVIDEND_ROUND_IDS = [9, 12].freeze
+
+  def self.perform(dry_run: true)
+    new(dry_run:).perform
+  end
+
+  def initialize(dry_run: true)
+    @dry_run = dry_run
+    @updated_count = 0
+    @skipped_count = 0
+    @already_set_count = 0
+    @zero_investment_entries = []
+  end
+
+  def perform
+    puts dry_run ? "DRY RUN MODE - No changes will be made" : "LIVE MODE - Dividends will be updated"
+    puts ""
+
+    dividend_rounds = DividendRound.where.not(id: SKIPPED_DIVIDEND_ROUND_IDS).order(:id)
+    puts "Processing #{dividend_rounds.count} dividend rounds (skipping IDs: #{SKIPPED_DIVIDEND_ROUND_IDS.join(', ')})..."
+    puts "================================================================================"
+
+    dividend_rounds.find_each do |dividend_round|
+      process_dividend_round(dividend_round)
+    end
+
+    print_summary
+  end
+
+  private
+    attr_reader :dry_run
+
+    def process_dividend_round(dividend_round)
+      puts "\n--- Dividend Round ##{dividend_round.id} (#{dividend_round.company.name}) ---"
+      puts "    Issued at: #{dividend_round.issued_at}"
+
+      dividend_round.dividends.includes(company_investor: [:share_holdings, :convertible_securities]).find_each do |dividend|
+        process_dividend(dividend, dividend_round)
+      end
+    end
+
+    def process_dividend(dividend, dividend_round)
+      if dividend.investment_amount_cents.present? && dividend.investment_amount_cents > 0
+        @already_set_count += 1
+        return
+      end
+
+      company_investor = dividend.company_investor
+      investment_amount = calculate_investment_amount(dividend, company_investor, dividend_round.issued_at)
+
+      if investment_amount == 0
+        @zero_investment_entries << { dividend_id: dividend.id, email: company_investor.user.email, dividend_round_id: dividend_round.id }
+      end
+
+      if dry_run
+        puts "    [DRY RUN] Dividend ##{dividend.id} (#{company_investor.user.email}): would set investment_amount_cents = #{investment_amount}"
+      else
+        dividend.update_column(:investment_amount_cents, investment_amount)
+        puts "    [UPDATED] Dividend ##{dividend.id} (#{company_investor.user.email}): investment_amount_cents = #{investment_amount}"
+      end
+      @updated_count += 1
+    end
+
+    def calculate_investment_amount(dividend, company_investor, issued_at)
+      if dividend.number_of_shares.present?
+        company_investor.share_holdings
+          .where("issued_at <= ?", issued_at)
+          .sum(:total_amount_in_cents)
+      else
+        company_investor.convertible_securities
+          .where("issued_at <= ?", issued_at)
+          .sum(:principal_value_in_cents)
+      end
+    end
+
+    def print_summary
+      puts "\n================================================================================"
+      puts "SUMMARY:"
+      puts "================================================================================"
+      puts "  Updated: #{@updated_count}"
+      puts "  Already set: #{@already_set_count}"
+      puts "  Zero investment: #{@zero_investment_entries.size}"
+      puts "  Skipped dividend rounds: #{SKIPPED_DIVIDEND_ROUND_IDS.join(', ')}"
+
+      if @zero_investment_entries.any?
+        puts "\n  Zero investment dividends:"
+        @zero_investment_entries.each do |entry|
+          puts "    - Dividend ##{entry[:dividend_id]} (DR ##{entry[:dividend_round_id]}, #{entry[:email]})"
+        end
+      end
+
+      if dry_run
+        puts "\n✓ Dry run complete! No changes were made."
+        puts "  To actually update, run with dry_run: false"
+      else
+        puts "\n✓ Backfill complete!"
+      end
+    end
+end
diff --git a/backend/spec/services/onetime/backfill_dividend_investment_amounts_spec.rb b/backend/spec/services/onetime/backfill_dividend_investment_amounts_spec.rb
new file mode 100644
index 0000000000..189b233c2d
--- /dev/null
+++ b/backend/spec/services/onetime/backfill_dividend_investment_amounts_spec.rb
@@ -0,0 +1,433 @@
+# frozen_string_literal: true
+
+RSpec.describe Onetime::BackfillDividendInvestmentAmounts do
+  let(:company) { create(:company) }
+
+  describe ".perform" do
+    context "with share holders only (e.g. Pylon)" do
+      let(:company_investor) { create(:company_investor, company:) }
+      let(:dividend_round) { create(:dividend_round, company:, issued_at: 1.day.ago) }
+      let!(:share_holding) do
+        create(:share_holding,
+               company_investor:,
+               issued_at: 1.month.ago,
+               number_of_shares: 100,
+               total_amount_in_cents: 50_000_00)
+      end
+      let!(:dividend) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round:,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 1_000_00)
+      end
+
+      it "sets investment_amount_cents from share_holdings" do
+        described_class.perform(dry_run: false)
+        expect(dividend.reload.investment_amount_cents).to eq(50_000_00)
+      end
+    end
+
+    context "with convertible holders only (e.g. Drink LMNT, Austin Flipsters, Fierce)" do
+      let(:company_investor) { create(:company_investor, company:) }
+      let(:dividend_round) { create(:dividend_round, company:, issued_at: 1.day.ago) }
+      let!(:convertible_security) do
+        create(:convertible_security,
+               company_investor:,
+               issued_at: 1.month.ago,
+               principal_value_in_cents: 25_000_00)
+      end
+      let!(:dividend) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round:,
+               number_of_shares: nil,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 500_00)
+      end
+
+      it "sets investment_amount_cents from convertible_securities" do
+        described_class.perform(dry_run: false)
+        expect(dividend.reload.investment_amount_cents).to eq(25_000_00)
+      end
+    end
+
+    context "with both share holders and convertible holders (e.g. Gumroad)" do
+      let(:share_investor) { create(:company_investor, company:) }
+      let(:convertible_investor) { create(:company_investor, company:) }
+      let(:both_investor) { create(:company_investor, company:) }
+      let(:dividend_round) { create(:dividend_round, company:, issued_at: 1.day.ago) }
+
+      let!(:share_holding) do
+        create(:share_holding,
+               company_investor: share_investor,
+               issued_at: 1.month.ago,
+               number_of_shares: 200,
+               total_amount_in_cents: 100_000_00)
+      end
+      let!(:convertible_security) do
+        create(:convertible_security,
+               company_investor: convertible_investor,
+               issued_at: 1.month.ago,
+               principal_value_in_cents: 10_000_00)
+      end
+      let!(:both_share_holding) do
+        create(:share_holding,
+               company_investor: both_investor,
+               issued_at: 1.month.ago,
+               number_of_shares: 50,
+               total_amount_in_cents: 25_000_00)
+      end
+      let!(:both_convertible_security) do
+        create(:convertible_security,
+               company_investor: both_investor,
+               issued_at: 1.month.ago,
+               principal_value_in_cents: 5_000_00)
+      end
+
+      let!(:share_dividend) do
+        create(:dividend,
+               company:,
+               company_investor: share_investor,
+               dividend_round:,
+               number_of_shares: 200,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 2_000_00)
+      end
+      let!(:convertible_dividend) do
+        create(:dividend,
+               company:,
+               company_investor: convertible_investor,
+               dividend_round:,
+               number_of_shares: nil,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 200_00)
+      end
+      # Investor with both shares and convertibles gets TWO dividends per round
+      let!(:both_shares_dividend) do
+        create(:dividend,
+               company:,
+               company_investor: both_investor,
+               dividend_round:,
+               number_of_shares: 50,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 300_00)
+      end
+      let!(:both_convertible_dividend) do
+        create(:dividend,
+               company:,
+               company_investor: both_investor,
+               dividend_round:,
+               number_of_shares: nil,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 200_00)
+      end
+
+      it "sets investment_amount_cents correctly for each type" do
+        described_class.perform(dry_run: false)
+
+        expect(share_dividend.reload.investment_amount_cents).to eq(100_000_00)
+        expect(convertible_dividend.reload.investment_amount_cents).to eq(10_000_00)
+        expect(both_shares_dividend.reload.investment_amount_cents).to eq(25_000_00)
+        expect(both_convertible_dividend.reload.investment_amount_cents).to eq(5_000_00)
+      end
+    end
+
+    context "with multiple share holdings and convertible securities per investor" do
+      let(:company_investor) { create(:company_investor, company:) }
+      let(:dividend_round) { create(:dividend_round, company:, issued_at: 1.day.ago) }
+
+      let!(:share_holding_1) do
+        create(:share_holding,
+               company_investor:,
+               issued_at: 6.months.ago,
+               number_of_shares: 100,
+               total_amount_in_cents: 10_000_00)
+      end
+      let!(:share_holding_2) do
+        create(:share_holding,
+               company_investor:,
+               issued_at: 3.months.ago,
+               number_of_shares: 50,
+               total_amount_in_cents: 7_500_00)
+      end
+      let!(:convertible_security_1) do
+        create(:convertible_security,
+               company_investor:,
+               issued_at: 6.months.ago,
+               principal_value_in_cents: 5_000_00)
+      end
+      let!(:convertible_security_2) do
+        create(:convertible_security,
+               company_investor:,
+               issued_at: 3.months.ago,
+               principal_value_in_cents: 3_000_00)
+      end
+
+      let!(:shares_dividend) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round:,
+               number_of_shares: 150,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 1_000_00)
+      end
+      let!(:convertible_dividend) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round:,
+               number_of_shares: nil,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 500_00)
+      end
+
+      it "sums all share holdings for shares dividend and all convertibles for convertible dividend" do
+        described_class.perform(dry_run: false)
+        expect(shares_dividend.reload.investment_amount_cents).to eq(17_500_00)
+        expect(convertible_dividend.reload.investment_amount_cents).to eq(8_000_00)
+      end
+    end
+
+    context "only includes holdings issued before the dividend round" do
+      let(:company_investor) { create(:company_investor, company:) }
+      let(:dividend_round) { create(:dividend_round, company:, issued_at: 2.months.ago) }
+
+      let!(:old_share_holding) do
+        create(:share_holding,
+               company_investor:,
+               issued_at: 6.months.ago,
+               number_of_shares: 100,
+               total_amount_in_cents: 10_000_00)
+      end
+      let!(:new_share_holding) do
+        create(:share_holding,
+               company_investor:,
+               issued_at: 1.month.ago,
+               number_of_shares: 200,
+               total_amount_in_cents: 20_000_00)
+      end
+      let!(:old_convertible) do
+        create(:convertible_security,
+               company_investor:,
+               issued_at: 6.months.ago,
+               principal_value_in_cents: 5_000_00)
+      end
+      let!(:new_convertible) do
+        create(:convertible_security,
+               company_investor:,
+               issued_at: 1.month.ago,
+               principal_value_in_cents: 8_000_00)
+      end
+
+      let!(:shares_dividend) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round:,
+               number_of_shares: 100,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 500_00)
+      end
+      let!(:convertible_dividend) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round:,
+               number_of_shares: nil,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 300_00)
+      end
+
+      it "only includes holdings issued before the dividend round" do
+        described_class.perform(dry_run: false)
+        expect(shares_dividend.reload.investment_amount_cents).to eq(10_000_00)
+        expect(convertible_dividend.reload.investment_amount_cents).to eq(5_000_00)
+      end
+    end
+
+    context "skips dividend rounds 9 and 12" do
+      let(:dividend_round_9) { create(:dividend_round, id: 9, company:, issued_at: 1.day.ago) }
+      let(:dividend_round_12) { create(:dividend_round, id: 12, company:, issued_at: 1.day.ago) }
+      let(:company_investor) { create(:company_investor, company:) }
+
+      let!(:convertible_security) do
+        create(:convertible_security,
+               company_investor:,
+               issued_at: 1.month.ago,
+               principal_value_in_cents: 10_000_00)
+      end
+
+      let!(:dividend_in_round_9) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round: dividend_round_9,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 100_00)
+      end
+      let!(:dividend_in_round_12) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round: dividend_round_12,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 200_00)
+      end
+
+      it "does not update dividends in skipped rounds" do
+        described_class.perform(dry_run: false)
+        expect(dividend_in_round_9.reload.investment_amount_cents).to be_nil
+        expect(dividend_in_round_12.reload.investment_amount_cents).to be_nil
+      end
+    end
+
+    context "skips dividends that already have investment_amount_cents" do
+      let(:company_investor) { create(:company_investor, company:) }
+      let(:dividend_round) { create(:dividend_round, company:, issued_at: 1.day.ago) }
+
+      let!(:share_holding) do
+        create(:share_holding,
+               company_investor:,
+               issued_at: 1.month.ago,
+               number_of_shares: 100,
+               total_amount_in_cents: 50_000_00)
+      end
+
+      let!(:dividend) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round:,
+               investment_amount_cents: 99_999_00,
+               total_amount_in_cents: 1_000_00)
+      end
+
+      it "does not overwrite existing investment_amount_cents" do
+        described_class.perform(dry_run: false)
+        expect(dividend.reload.investment_amount_cents).to eq(99_999_00)
+      end
+    end
+
+    context "when calculated investment is zero" do
+      let(:company_investor) { create(:company_investor, company:) }
+      let(:dividend_round) { create(:dividend_round, company:, issued_at: 1.day.ago) }
+
+      let!(:dividend) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round:,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 100_00)
+      end
+
+      it "still updates investment_amount_cents to zero with a warning" do
+        described_class.perform(dry_run: false)
+        expect(dividend.reload.investment_amount_cents).to eq(0)
+      end
+    end
+
+    context "handles investment_amount_cents set to 0 (local DB NOT NULL constraint)" do
+      let(:company_investor) { create(:company_investor, company:) }
+      let(:dividend_round) { create(:dividend_round, company:, issued_at: 1.day.ago) }
+
+      let!(:share_holding) do
+        create(:share_holding,
+               company_investor:,
+               issued_at: 1.month.ago,
+               number_of_shares: 100,
+               total_amount_in_cents: 50_000_00)
+      end
+
+      let!(:dividend) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round:,
+               investment_amount_cents: 0,
+               total_amount_in_cents: 1_000_00)
+      end
+
+      it "backfills dividends with investment_amount_cents of 0" do
+        described_class.perform(dry_run: false)
+        expect(dividend.reload.investment_amount_cents).to eq(50_000_00)
+      end
+    end
+
+    context "dry run mode" do
+      let(:company_investor) { create(:company_investor, company:) }
+      let(:dividend_round) { create(:dividend_round, company:, issued_at: 1.day.ago) }
+
+      let!(:share_holding) do
+        create(:share_holding,
+               company_investor:,
+               issued_at: 1.month.ago,
+               number_of_shares: 100,
+               total_amount_in_cents: 50_000_00)
+      end
+
+      let!(:dividend) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round:,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 1_000_00)
+      end
+
+      it "does not update any records" do
+        described_class.perform(dry_run: true)
+        expect(dividend.reload.investment_amount_cents).to be_nil
+      end
+    end
+
+    context "with multiple dividend rounds for the same company" do
+      let(:company_investor) { create(:company_investor, company:) }
+      let(:round_1) { create(:dividend_round, company:, issued_at: 6.months.ago) }
+      let(:round_2) { create(:dividend_round, company:, issued_at: 1.day.ago) }
+
+      let!(:old_share_holding) do
+        create(:share_holding,
+               company_investor:,
+               issued_at: 1.year.ago,
+               number_of_shares: 100,
+               total_amount_in_cents: 10_000_00)
+      end
+      let!(:new_share_holding) do
+        create(:share_holding,
+               company_investor:,
+               issued_at: 3.months.ago,
+               number_of_shares: 200,
+               total_amount_in_cents: 30_000_00)
+      end
+
+      let!(:dividend_round_1) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round: round_1,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 500_00)
+      end
+      let!(:dividend_round_2) do
+        create(:dividend,
+               company:,
+               company_investor:,
+               dividend_round: round_2,
+               investment_amount_cents: nil,
+               total_amount_in_cents: 1_000_00)
+      end
+
+      it "calculates correct historical investment amount for each round" do
+        described_class.perform(dry_run: false)
+
+        expect(dividend_round_1.reload.investment_amount_cents).to eq(10_000_00)
+        expect(dividend_round_2.reload.investment_amount_cents).to eq(40_000_00)
+      end
+    end
+  end
+end