diff --git a/modules/structs/repo_branch.go b/modules/structs/repo_branch.go
index 55c98d60b9..5416f43b0d 100644
--- a/modules/structs/repo_branch.go
+++ b/modules/structs/repo_branch.go
@@ -136,6 +136,7 @@ type UpdateBranchProtectionPriories struct {
 
 type MergeUpstreamRequest struct {
 	Branch string `json:"branch"`
+	FfOnly bool   `json:"ff_only"`
 }
 
 type MergeUpstreamResponse struct {
diff --git a/routers/api/v1/repo/branch.go b/routers/api/v1/repo/branch.go
index fe82550fdd..6e496e6fd3 100644
--- a/routers/api/v1/repo/branch.go
+++ b/routers/api/v1/repo/branch.go
@@ -1181,7 +1181,7 @@ func MergeUpstream(ctx *context.APIContext) {
 	//   "404":
 	//     "$ref": "#/responses/notFound"
 	form := web.GetForm(ctx).(*api.MergeUpstreamRequest)
-	mergeStyle, err := repo_service.MergeUpstream(ctx, ctx.Doer, ctx.Repo.Repository, form.Branch)
+	mergeStyle, err := repo_service.MergeUpstream(ctx, ctx.Doer, ctx.Repo.Repository, form.Branch, form.FfOnly)
 	if err != nil {
 		if errors.Is(err, util.ErrInvalidArgument) {
 			ctx.APIError(http.StatusBadRequest, err)
diff --git a/routers/web/repo/branch.go b/routers/web/repo/branch.go
index dc8a90b2ae..96d1d87836 100644
--- a/routers/web/repo/branch.go
+++ b/routers/web/repo/branch.go
@@ -258,7 +258,7 @@ func CreateBranch(ctx *context.Context) {
 
 func MergeUpstream(ctx *context.Context) {
 	branchName := ctx.FormString("branch")
-	_, err := repo_service.MergeUpstream(ctx, ctx.Doer, ctx.Repo.Repository, branchName)
+	_, err := repo_service.MergeUpstream(ctx, ctx.Doer, ctx.Repo.Repository, branchName, false)
 	if err != nil {
 		if errors.Is(err, util.ErrNotExist) {
 			ctx.JSONErrorNotFound()
diff --git a/services/repository/merge_upstream.go b/services/repository/merge_upstream.go
index 34e01df723..8d6f11372c 100644
--- a/services/repository/merge_upstream.go
+++ b/services/repository/merge_upstream.go
@@ -18,7 +18,7 @@ import (
 )
 
 // MergeUpstream merges the base repository's default branch into the fork repository's current branch.
-func MergeUpstream(ctx reqctx.RequestContext, doer *user_model.User, repo *repo_model.Repository, branch string) (mergeStyle string, err error) {
+func MergeUpstream(ctx reqctx.RequestContext, doer *user_model.User, repo *repo_model.Repository, branch string, ffOnly bool) (mergeStyle string, err error) {
 	if err = repo.MustNotBeArchived(); err != nil {
 		return "", err
 	}
@@ -45,6 +45,11 @@ func MergeUpstream(ctx reqctx.RequestContext, doer *user_model.User, repo *repo_
 		return "", err
 	}
 
+	// If ff_only is requested and fast-forward failed, return error
+	if ffOnly {
+		return "", util.NewInvalidArgumentErrorf("fast-forward merge not possible: branch has diverged")
+	}
+
 	// TODO: FakePR: it is somewhat hacky, but it is the only way to "merge" at the moment
 	// ideally in the future the "merge" functions should be refactored to decouple from the PullRequest
 	fakeIssue := &issue_model.Issue{
diff --git a/templates/swagger/v1_json.tmpl b/templates/swagger/v1_json.tmpl
index 59115431b9..4fa72e16d5 100644
--- a/templates/swagger/v1_json.tmpl
+++ b/templates/swagger/v1_json.tmpl
@@ -24839,6 +24839,10 @@
         "branch": {
           "type": "string",
           "x-go-name": "Branch"
+        },
+        "ff_only": {
+          "type": "boolean",
+          "x-go-name": "FfOnly"
         }
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"
diff --git a/tests/integration/repo_merge_upstream_test.go b/tests/integration/repo_merge_upstream_test.go
index e928b04e9b..d33d31c646 100644
--- a/tests/integration/repo_merge_upstream_test.go
+++ b/tests/integration/repo_merge_upstream_test.go
@@ -147,5 +147,37 @@ func TestRepoMergeUpstream(t *testing.T) {
 				return queryMergeUpstreamButtonLink(htmlDoc) == ""
 			}, 5*time.Second, 100*time.Millisecond)
 		})
+
+		t.Run("FastForwardOnly", func(t *testing.T) {
+			// Create a clean branch for fast-forward testing
+			req = NewRequestWithValues(t, "POST", fmt.Sprintf("/%s/test-repo-fork/branches/_new/branch/master", forkUser.Name), map[string]string{
+				"_csrf":           GetUserCSRFToken(t, session),
+				"new_branch_name": "ff-test-branch",
+			})
+			session.MakeRequest(t, req, http.StatusSeeOther)
+
+			// Add content to base repository that can be fast-forwarded
+			require.NoError(t, createOrReplaceFileInBranch(baseUser, baseRepo, "ff-test.txt", "master", "ff-content-1"))
+
+			// ff_only=true with fast-forward possible (should succeed)
+			req = NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/test-repo-fork/merge-upstream", forkUser.Name), &api.MergeUpstreamRequest{
+				Branch: "ff-test-branch",
+				FfOnly: true,
+			}).AddTokenAuth(token)
+			resp := MakeRequest(t, req, http.StatusOK)
+
+			var mergeResp api.MergeUpstreamResponse
+			DecodeJSON(t, resp, &mergeResp)
+			assert.Equal(t, "fast-forward", mergeResp.MergeStyle)
+
+			// ff_only=true when fast-forward is not possible (should fail)
+			require.NoError(t, createOrReplaceFileInBranch(baseUser, baseRepo, "another-file.txt", "master", "more-content"))
+
+			req = NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/test-repo-fork/merge-upstream", forkUser.Name), &api.MergeUpstreamRequest{
+				Branch: "fork-branch",
+				FfOnly: true,
+			}).AddTokenAuth(token)
+			MakeRequest(t, req, http.StatusBadRequest)
+		})
 	})
 }