diff --git a/app-modules/squads/database/migrations/2026_07_30_113028_add_single_captain_unique_to_squad_members.php b/app-modules/squads/database/migrations/2026_07_30_113028_add_single_captain_unique_to_squad_members.php
new file mode 100644
index 00000000..6bd761bb
--- /dev/null
+++ b/app-modules/squads/database/migrations/2026_07_30_113028_add_single_captain_unique_to_squad_members.php
@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\DB;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        DB::statement(
+            "CREATE UNIQUE INDEX squad_members_squad_id_captain_unique ON squad_members (squad_id) WHERE role = 'captain'"
+        );
+    }
+
+    public function down(): void
+    {
+        DB::statement('DROP INDEX IF EXISTS squad_members_squad_id_captain_unique');
+    }
+};
diff --git a/app-modules/squads/src/Actions/AssignCaptain.php b/app-modules/squads/src/Actions/AssignCaptain.php
new file mode 100644
index 00000000..83bfc0c1
--- /dev/null
+++ b/app-modules/squads/src/Actions/AssignCaptain.php
@@ -0,0 +1,96 @@
+<?php
+
+declare(strict_types=1);
+
+namespace He4rt\Squads\Actions;
+
+use He4rt\Identity\User\Models\User;
+use He4rt\Squads\Enums\MembershipAction;
+use He4rt\Squads\Enums\SquadRole;
+use He4rt\Squads\Exceptions\NotAnActiveSquadMember;
+use He4rt\Squads\Models\Squad;
+use He4rt\Squads\Models\SquadMember;
+use Illuminate\Auth\Access\AuthorizationException;
+use Illuminate\Support\Facades\DB;
+
+/**
+ * Records the outcome of an off-system captain election.
+ *
+ * The seat is single: assigning a new captain demotes the incumbent to
+ * `Member` in the same transaction, so the partial unique index
+ * `UNIQUE (squad_id) WHERE role = 'captain'` never sees two captains.
+ *
+ * Vacating the seat is not this Action's job — a captain who leaves becomes
+ * an `ExMember` (see `MarkExMember`), which frees the seat by itself.
+ */
+final readonly class AssignCaptain
+{
+    public function __construct(
+        private RecordMembershipEvent $recordMembershipEvent,
+    ) {}
+
+    public function handle(User $actor, Squad $squad, User $subject, ?string $reason = null): SquadMember
+    {
+        throw_unless($actor->isAdmin(), AuthorizationException::class);
+
+        return DB::transaction(function () use ($squad, $subject, $actor, $reason): SquadMember {
+            // Locking the subject's row serializes concurrent assignments of the same
+            // person: the loser re-reads the incumbent after the winner commits and
+            // short-circuits below, instead of appending a phantom pair of events.
+            $member = SquadMember::query()
+                ->where('squad_id', $squad->id)
+                ->where('user_id', $subject->id)
+                ->whereNot('role', SquadRole::ExMember)
+                ->lockForUpdate()
+                ->first();
+
+            throw_if($member === null, NotAnActiveSquadMember::for($squad, $subject));
+
+            $incumbent = $squad->captain()->first();
+
+            if ($incumbent?->is($member)) {
+                return $member;
+            }
+
+            if ($incumbent !== null) {
+                $this->demote($actor, $squad, $incumbent, $reason);
+            }
+
+            $fromRole = $member->role;
+            $member->update([
+                'role' => SquadRole::Captain,
+            ]);
+
+            $this->recordMembershipEvent->handle(
+                squad: $squad,
+                subject: $subject,
+                action: MembershipAction::CaptainAssigned,
+                fromRole: $fromRole,
+                toRole: SquadRole::Captain,
+                actor: $actor,
+                reason: $reason,
+            );
+
+            return $member->refresh();
+        });
+    }
+
+    private function demote(User $actor, Squad $squad, SquadMember $incumbent, ?string $reason): void
+    {
+        $fromRole = $incumbent->role;
+
+        $incumbent->update([
+            'role' => SquadRole::Member,
+        ]);
+
+        $this->recordMembershipEvent->handle(
+            squad: $squad,
+            subject: $incumbent->user,
+            action: MembershipAction::Demote,
+            fromRole: $fromRole,
+            toRole: SquadRole::Member,
+            actor: $actor,
+            reason: $reason,
+        );
+    }
+}
diff --git a/app-modules/squads/src/Enums/MembershipAction.php b/app-modules/squads/src/Enums/MembershipAction.php
index a38d1e15..6d603dcd 100644
--- a/app-modules/squads/src/Enums/MembershipAction.php
+++ b/app-modules/squads/src/Enums/MembershipAction.php
@@ -10,4 +10,5 @@ enum MembershipAction: string
     case Leave = 'leave';
     case Promote = 'promote';
     case Demote = 'demote';
+    case CaptainAssigned = 'captain_assigned';
 }
diff --git a/app-modules/squads/src/Exceptions/NotAnActiveSquadMember.php b/app-modules/squads/src/Exceptions/NotAnActiveSquadMember.php
new file mode 100644
index 00000000..d9a6d21d
--- /dev/null
+++ b/app-modules/squads/src/Exceptions/NotAnActiveSquadMember.php
@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace He4rt\Squads\Exceptions;
+
+use Exception;
+use He4rt\Identity\User\Models\User;
+use He4rt\Squads\Models\Squad;
+
+final class NotAnActiveSquadMember extends Exception
+{
+    public static function for(Squad $squad, User $subject): self
+    {
+        return new self(
+            sprintf('User "%s" holds no active membership in squad "%s".', $subject->id, $squad->id)
+        );
+    }
+}
diff --git a/app-modules/squads/src/Models/Squad.php b/app-modules/squads/src/Models/Squad.php
index f17a127d..cd45ad04 100644
--- a/app-modules/squads/src/Models/Squad.php
+++ b/app-modules/squads/src/Models/Squad.php
@@ -6,12 +6,14 @@
 
 use Carbon\CarbonInterface;
 use He4rt\Squads\Database\Factories\SquadFactory;
+use He4rt\Squads\Enums\SquadRole;
 use He4rt\Squads\Enums\SquadStatus;
 use Illuminate\Database\Eloquent\Attributes\Table;
 use Illuminate\Database\Eloquent\Attributes\UseFactory;
 use Illuminate\Database\Eloquent\Concerns\HasUuids;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\HasOne;
 
 /**
  * @property string $id
@@ -37,6 +39,18 @@ final class Squad extends Model
         'status',
     ];
 
+    /**
+     * The squad's captain seat, derived from the pivot — there is no
+     * denormalized `captain_id`. A vacant seat resolves to `null`.
+     *
+     * @return HasOne<SquadMember, $this>
+     */
+    public function captain(): HasOne
+    {
+        return $this->hasOne(SquadMember::class)
+            ->where('role', SquadRole::Captain);
+    }
+
     /**
      * @return array<string, string>
      */
diff --git a/app-modules/squads/tests/Feature/AssignCaptainTest.php b/app-modules/squads/tests/Feature/AssignCaptainTest.php
new file mode 100644
index 00000000..fa12a023
--- /dev/null
+++ b/app-modules/squads/tests/Feature/AssignCaptainTest.php
@@ -0,0 +1,163 @@
+<?php
+
+declare(strict_types=1);
+
+use He4rt\Identity\User\Models\User;
+use He4rt\Squads\Actions\AssignCaptain;
+use He4rt\Squads\Enums\MembershipAction;
+use He4rt\Squads\Enums\SquadRole;
+use He4rt\Squads\Exceptions\NotAnActiveSquadMember;
+use He4rt\Squads\Models\Squad;
+use He4rt\Squads\Models\SquadMember;
+use He4rt\Squads\Models\SquadMembershipEvent;
+use Illuminate\Auth\Access\AuthorizationException;
+
+beforeEach(function (): void {
+    config(['he4rt.admins' => 'guisaliba']);
+
+    $this->admin = User::factory()->create([
+        'username' => 'guisaliba',
+    ]);
+});
+
+test('super-admin assigns a member as captain and the trail records it', function (): void {
+    $squad = Squad::factory()->create();
+    $subject = User::factory()->create();
+
+    SquadMember::factory()->create([
+        'squad_id' => $squad->id,
+        'user_id' => $subject->id,
+        'role' => SquadRole::Member,
+    ]);
+
+    expect($squad->captain()->first())->toBeNull();
+
+    $member = resolve(AssignCaptain::class)->handle(
+        actor: $this->admin,
+        squad: $squad,
+        subject: $subject,
+        reason: 'Elected off-system.',
+    );
+
+    expect($member->role)->toBe(SquadRole::Captain)
+        ->and($squad->captain()->first()->user_id)->toBe($subject->id);
+
+    $this->assertDatabaseHas('squad_membership_events', [
+        'squad_id' => $squad->id,
+        'user_id' => $subject->id,
+        'actor_id' => $this->admin->id,
+        'action' => MembershipAction::CaptainAssigned->value,
+        'from_role' => SquadRole::Member->value,
+        'to_role' => SquadRole::Captain->value,
+        'reason' => 'Elected off-system.',
+    ]);
+});
+
+test('assigning a new captain demotes the incumbent and leaves a single captain', function (): void {
+    $squad = Squad::factory()->create();
+    $incumbent = User::factory()->create();
+    $successor = User::factory()->create();
+
+    SquadMember::factory()->create([
+        'squad_id' => $squad->id,
+        'user_id' => $incumbent->id,
+        'role' => SquadRole::Captain,
+    ]);
+    SquadMember::factory()->create([
+        'squad_id' => $squad->id,
+        'user_id' => $successor->id,
+        'role' => SquadRole::SubCaptain,
+    ]);
+
+    resolve(AssignCaptain::class)->handle(
+        actor: $this->admin,
+        squad: $squad,
+        subject: $successor,
+    );
+
+    $captains = SquadMember::query()
+        ->where('squad_id', $squad->id)
+        ->where('role', SquadRole::Captain)
+        ->get();
+
+    expect($captains)->toHaveCount(1)
+        ->and($captains->first()->user_id)->toBe($successor->id)
+        ->and($squad->captain()->first()->user_id)->toBe($successor->id);
+
+    $this->assertDatabaseHas('squad_members', [
+        'squad_id' => $squad->id,
+        'user_id' => $incumbent->id,
+        'role' => SquadRole::Member->value,
+    ]);
+
+    $this->assertDatabaseHas('squad_membership_events', [
+        'squad_id' => $squad->id,
+        'user_id' => $incumbent->id,
+        'actor_id' => $this->admin->id,
+        'action' => MembershipAction::Demote->value,
+        'from_role' => SquadRole::Captain->value,
+        'to_role' => SquadRole::Member->value,
+    ]);
+
+    $this->assertDatabaseHas('squad_membership_events', [
+        'squad_id' => $squad->id,
+        'user_id' => $successor->id,
+        'action' => MembershipAction::CaptainAssigned->value,
+        'from_role' => SquadRole::SubCaptain->value,
+        'to_role' => SquadRole::Captain->value,
+    ]);
+});
+
+test('re-assigning the current captain records nothing', function (): void {
+    $squad = Squad::factory()->create();
+    $captain = User::factory()->create();
+
+    SquadMember::factory()->create([
+        'squad_id' => $squad->id,
+        'user_id' => $captain->id,
+        'role' => SquadRole::Captain,
+    ]);
+
+    resolve(AssignCaptain::class)->handle(
+        actor: $this->admin,
+        squad: $squad,
+        subject: $captain,
+    );
+
+    expect(SquadMembershipEvent::query()->where('squad_id', $squad->id)->count())->toBe(0)
+        ->and($squad->captain()->first()->user_id)->toBe($captain->id);
+});
+
+test('a person without an active membership cannot be assigned captain', function (): void {
+    $squad = Squad::factory()->create();
+    $outsider = User::factory()->create();
+
+    SquadMember::factory()->create([
+        'squad_id' => $squad->id,
+        'user_id' => $outsider->id,
+        'role' => SquadRole::ExMember,
+    ]);
+
+    resolve(AssignCaptain::class)->handle(
+        actor: $this->admin,
+        squad: $squad,
+        subject: $outsider,
+    );
+})->throws(NotAnActiveSquadMember::class);
+
+test('common user cannot assign a captain', function (): void {
+    $squad = Squad::factory()->create();
+    $subject = User::factory()->create();
+
+    SquadMember::factory()->create([
+        'squad_id' => $squad->id,
+        'user_id' => $subject->id,
+        'role' => SquadRole::Member,
+    ]);
+
+    resolve(AssignCaptain::class)->handle(
+        actor: User::factory()->create(['username' => 'common-user']),
+        squad: $squad,
+        subject: $subject,
+    );
+})->throws(AuthorizationException::class);
diff --git a/app-modules/squads/tests/Feature/SquadMemberTest.php b/app-modules/squads/tests/Feature/SquadMemberTest.php
index 77f50f42..dcdb1f9a 100644
--- a/app-modules/squads/tests/Feature/SquadMemberTest.php
+++ b/app-modules/squads/tests/Feature/SquadMemberTest.php
@@ -42,3 +42,17 @@
         'user_id' => $user->id,
     ]);
 })->throws(QueryException::class);
+
+test('a squad cannot hold two captains', function (): void {
+    $squad = Squad::factory()->create();
+
+    SquadMember::factory()->create([
+        'squad_id' => $squad->id,
+        'role' => SquadRole::Captain,
+    ]);
+
+    SquadMember::factory()->create([
+        'squad_id' => $squad->id,
+        'role' => SquadRole::Captain,
+    ]);
+})->throws(QueryException::class);