diff --git a/cmd/server.go b/cmd/server.go
index 6f8875e..99fa1db 100644
--- a/cmd/server.go
+++ b/cmd/server.go
@@ -152,6 +152,11 @@ func main() {
 	// Storage module - provides R2 storage integration for images
 	storageDeps := storage.ProvideStorageDependencies(appRouter)
 
+	// Set storage port for contest thumbnail upload
+	if storageDeps != nil {
+		contestDeps.ContestService.SetStoragePort(storageDeps.StoragePort)
+	}
+
 	// Banner module - provides main banner management for homepage
 	bannerDeps := banner.ProvideBannerDependencies(db, appRouter)
 
diff --git a/internal/contest/application/contest_service.go b/internal/contest/application/contest_service.go
index 963e4fd..e6eb9e6 100644
--- a/internal/contest/application/contest_service.go
+++ b/internal/contest/application/contest_service.go
@@ -10,10 +10,15 @@ import (
 	commonDto "github.com/FOR-GAMERS/GAMERS-BE/internal/global/common/dto"
 	"github.com/FOR-GAMERS/GAMERS-BE/internal/global/exception"
 	oauth2Port "github.com/FOR-GAMERS/GAMERS-BE/internal/oauth2/application/port"
+	storageDomain "github.com/FOR-GAMERS/GAMERS-BE/internal/storage/domain"
 	"context"
 	"errors"
+	"fmt"
 	"log"
+	"mime/multipart"
 	"time"
+
+	"github.com/google/uuid"
 )
 
 // TournamentGeneratorPort defines the interface for tournament generation
@@ -32,6 +37,7 @@ type ContestService struct {
 	tournamentGenerator   TournamentGeneratorPort
 	teamDBPort            gamePort.TeamDatabasePort
 	gameTeamDBPort        gamePort.GameTeamDatabasePort
+	storagePort           port.ContestStoragePort
 }
 
 func NewContestService(
@@ -94,6 +100,11 @@ func NewContestServiceFull(
 	}
 }
 
+// SetStoragePort sets the storage port for file upload operations
+func (c *ContestService) SetStoragePort(storagePort port.ContestStoragePort) {
+	c.storagePort = storagePort
+}
+
 func (c *ContestService) SaveContest(req *dto.CreateContestRequest, userId int64) (*domain.Contest, *dto.DiscordLinkRequiredResponse, error) {
 	// Check if user has linked Discord account
 	discordAccount, err := c.oauth2Repository.FindDiscordAccountByUserId(userId)
@@ -470,3 +481,68 @@ func (c *ContestService) GetDiscordTextChannels(guildID string) ([]port.DiscordC
 	}
 	return c.discordValidator.GetGuildTextChannels(guildID)
 }
+
+// UploadThumbnail uploads a thumbnail image for a contest and updates the contest record
+func (c *ContestService) UploadThumbnail(ctx context.Context, contestId, userId int64, file *multipart.FileHeader) (*dto.ThumbnailUploadResponse, error) {
+	if c.storagePort == nil {
+		return nil, exception.ErrStorageUploadFailed
+	}
+
+	// Check leader permission
+	if err := c.checkLeaderPermission(contestId, userId); err != nil {
+		return nil, err
+	}
+
+	// Validate file
+	if err := storageDomain.ValidateFile(file, storageDomain.UploadTypeContestThumbnail); err != nil {
+		return nil, err
+	}
+
+	// Generate storage key
+	mimeType := file.Header.Get("Content-Type")
+	ext := storageDomain.GetExtensionFromMimeType(mimeType)
+	key := fmt.Sprintf("%s/%d/%s%s", storageDomain.UploadTypeContestThumbnail, contestId, uuid.New().String(), ext)
+
+	// Upload file to storage
+	src, err := file.Open()
+	if err != nil {
+		return nil, exception.ErrStorageUploadFailed
+	}
+	defer src.Close()
+
+	if err := c.storagePort.Upload(ctx, key, src, file.Size, mimeType); err != nil {
+		return nil, exception.ErrStorageUploadFailed
+	}
+
+	// Get public URL and update contest record
+	url := c.storagePort.GetPublicURL(key)
+
+	contest, err := c.repository.GetContestById(contestId)
+	if err != nil {
+		return nil, err
+	}
+
+	// Delete old thumbnail from storage if exists
+	if contest.BannerKey != nil && *contest.BannerKey != "" {
+		go func() {
+			if delErr := c.storagePort.Delete(context.Background(), *contest.BannerKey); delErr != nil {
+				log.Printf("[UploadThumbnail] Failed to delete old thumbnail %s: %v", *contest.BannerKey, delErr)
+			}
+		}()
+	}
+
+	contest.Thumbnail = &url
+	contest.BannerKey = &key
+
+	if err := c.repository.UpdateContest(contest); err != nil {
+		return nil, err
+	}
+
+	return &dto.ThumbnailUploadResponse{
+		Key:        key,
+		URL:        url,
+		Size:       file.Size,
+		MimeType:   mimeType,
+		UploadedAt: time.Now(),
+	}, nil
+}
diff --git a/internal/contest/application/dto/contest_dto.go b/internal/contest/application/dto/contest_dto.go
index 2d017a7..f9d73c7 100644
--- a/internal/contest/application/dto/contest_dto.go
+++ b/internal/contest/application/dto/contest_dto.go
@@ -269,6 +269,15 @@ func ToMyContestResponses(contests []*port.ContestWithMembership) []*MyContestRe
 	return responses
 }
 
+// ThumbnailUploadResponse represents the response after uploading a contest thumbnail
+type ThumbnailUploadResponse struct {
+	Key        string    `json:"key"`
+	URL        string    `json:"url"`
+	Size       int64     `json:"size"`
+	MimeType   string    `json:"mime_type"`
+	UploadedAt time.Time `json:"uploaded_at"`
+}
+
 // ChangeMemberRoleRequest represents the request to change a member's role
 type ChangeMemberRoleRequest struct {
 	MemberType domain.MemberType `json:"member_type" binding:"required"`
diff --git a/internal/contest/application/port/contest_storage_port.go b/internal/contest/application/port/contest_storage_port.go
new file mode 100644
index 0000000..02c7e69
--- /dev/null
+++ b/internal/contest/application/port/contest_storage_port.go
@@ -0,0 +1,12 @@
+package port
+
+import (
+	"context"
+	"io"
+)
+
+type ContestStoragePort interface {
+	Upload(ctx context.Context, key string, body io.Reader, size int64, contentType string) error
+	Delete(ctx context.Context, key string) error
+	GetPublicURL(key string) string
+}
diff --git a/internal/contest/presentation/contest_controller.go b/internal/contest/presentation/contest_controller.go
index e8c6ab5..f886af3 100644
--- a/internal/contest/presentation/contest_controller.go
+++ b/internal/contest/presentation/contest_controller.go
@@ -37,6 +37,7 @@ func (c *ContestController) RegisterRoute() {
 	privateGroup.GET("/me", c.GetMyContests)
 	privateGroup.PATCH("/:id", c.UpdateContest)
 	privateGroup.DELETE("/:id", c.DeleteContest)
+	privateGroup.POST("/:id/thumbnail", c.UploadThumbnail)
 	privateGroup.POST("/:id/start", c.StartContest)
 	privateGroup.POST("/:id/stop", c.StopContest)
 
@@ -263,6 +264,43 @@ func (c *ContestController) StopContest(ctx *gin.Context) {
 	c.helper.RespondOK(ctx, contest, err, "contest stopped successfully")
 }
 
+// UploadThumbnail godoc
+// @Summary Upload a contest thumbnail image
+// @Description Upload a thumbnail image for a contest. Maximum file size is 5MB. Allowed formats: jpeg, png, webp. Only the contest leader can upload.
+// @Tags contests
+// @Accept multipart/form-data
+// @Produce json
+// @Security BearerAuth
+// @Param id path int true "Contest ID"
+// @Param file formData file true "Image file (max 5MB, jpeg/png/webp)"
+// @Success 201 {object} response.Response{data=dto.ThumbnailUploadResponse}
+// @Failure 400 {object} response.Response
+// @Failure 401 {object} response.Response
+// @Failure 403 {object} response.Response
+// @Router /api/contests/{id}/thumbnail [post]
+func (c *ContestController) UploadThumbnail(ctx *gin.Context) {
+	id, err := strconv.ParseInt(ctx.Param("id"), 10, 64)
+	if err != nil {
+		response.JSON(ctx, response.BadRequest("invalid contest id"))
+		return
+	}
+
+	userId, ok := middleware.GetUserIdFromContext(ctx)
+	if !ok {
+		response.JSON(ctx, response.Error(401, "user not authenticated"))
+		return
+	}
+
+	file, err := ctx.FormFile("file")
+	if err != nil {
+		response.JSON(ctx, response.BadRequest("file is required"))
+		return
+	}
+
+	result, err := c.service.UploadThumbnail(ctx.Request.Context(), id, userId, file)
+	c.helper.RespondCreated(ctx, result, err, "thumbnail uploaded successfully")
+}
+
 // GetMyContests godoc
 // @Summary Get contests I have joined
 // @Description Get all contests that the authenticated user has joined with pagination, sorting, and filtering support
@@ -421,3 +459,26 @@ func HandleStopContest(ctx *gin.Context, service *application.ContestService, he
 	contest, err := service.StopContest(ctx.Request.Context(), id, userId)
 	helper.RespondOK(ctx, contest, err, "contest stopped successfully")
 }
+
+func HandleUploadThumbnail(ctx *gin.Context, service *application.ContestService, helper *handler.ControllerHelper) {
+	id, err := strconv.ParseInt(ctx.Param("id"), 10, 64)
+	if err != nil {
+		response.JSON(ctx, response.BadRequest("invalid contest id"))
+		return
+	}
+
+	userId, ok := middleware.GetUserIdFromContext(ctx)
+	if !ok {
+		response.JSON(ctx, response.Error(401, "user not authenticated"))
+		return
+	}
+
+	file, err := ctx.FormFile("file")
+	if err != nil {
+		response.JSON(ctx, response.BadRequest("file is required"))
+		return
+	}
+
+	result, err := service.UploadThumbnail(ctx.Request.Context(), id, userId, file)
+	helper.RespondCreated(ctx, result, err, "thumbnail uploaded successfully")
+}
diff --git a/internal/storage/domain/storage.go b/internal/storage/domain/storage.go
index 7a4c569..7dbb577 100644
--- a/internal/storage/domain/storage.go
+++ b/internal/storage/domain/storage.go
@@ -11,15 +11,17 @@ import (
 type UploadType string
 
 const (
-	UploadTypeContestBanner UploadType = "contest-banners"
-	UploadTypeUserProfile   UploadType = "user-profiles"
-	UploadTypeMainBanner    UploadType = "main-banners"
+	UploadTypeContestBanner    UploadType = "contest-banners"
+	UploadTypeContestThumbnail UploadType = "contest-thumbnails"
+	UploadTypeUserProfile      UploadType = "user-profiles"
+	UploadTypeMainBanner       UploadType = "main-banners"
 )
 
 const (
-	MaxContestBannerSize = 5 * 1024 * 1024 // 5MB
-	MaxUserProfileSize   = 2 * 1024 * 1024 // 2MB
-	MaxMainBannerSize    = 5 * 1024 * 1024 // 5MB
+	MaxContestBannerSize    = 5 * 1024 * 1024 // 5MB
+	MaxContestThumbnailSize = 5 * 1024 * 1024 // 5MB
+	MaxUserProfileSize      = 2 * 1024 * 1024 // 2MB
+	MaxMainBannerSize       = 5 * 1024 * 1024 // 5MB
 )
 
 var AllowedMimeTypes = map[string]bool{
@@ -74,6 +76,8 @@ func getMaxSize(uploadType UploadType) int64 {
 	switch uploadType {
 	case UploadTypeContestBanner:
 		return MaxContestBannerSize
+	case UploadTypeContestThumbnail:
+		return MaxContestThumbnailSize
 	case UploadTypeUserProfile:
 		return MaxUserProfileSize
 	case UploadTypeMainBanner:
diff --git a/internal/storage/provider.go b/internal/storage/provider.go
index e153fdf..d9743c4 100644
--- a/internal/storage/provider.go
+++ b/internal/storage/provider.go
@@ -4,6 +4,7 @@ import (
 	"github.com/FOR-GAMERS/GAMERS-BE/internal/global/common/handler"
 	"github.com/FOR-GAMERS/GAMERS-BE/internal/global/common/router"
 	"github.com/FOR-GAMERS/GAMERS-BE/internal/storage/application"
+	"github.com/FOR-GAMERS/GAMERS-BE/internal/storage/application/port"
 	"github.com/FOR-GAMERS/GAMERS-BE/internal/storage/infra"
 	"github.com/FOR-GAMERS/GAMERS-BE/internal/storage/presentation"
 	"log"
@@ -12,6 +13,7 @@ import (
 type Dependencies struct {
 	Controller     *presentation.StorageController
 	StorageService *application.StorageService
+	StoragePort    port.StoragePort
 }
 
 func ProvideStorageDependencies(router *router.Router) *Dependencies {
@@ -31,5 +33,6 @@ func ProvideStorageDependencies(router *router.Router) *Dependencies {
 	return &Dependencies{
 		Controller:     storageController,
 		StorageService: storageService,
+		StoragePort:    storageAdapter,
 	}
 }