diff --git a/models/issues/pull.go b/models/issues/pull.go
index 7d299eac2..98d161738 100644
--- a/models/issues/pull.go
+++ b/models/issues/pull.go
@@ -922,7 +922,14 @@ func PullRequestCodeOwnersReview(ctx context.Context, pull *Issue, pr *PullReque
 	}
 
 	rules, _ := GetCodeOwnersFromContent(ctx, data)
-	changedFiles, err := repo.GetFilesChangedBetween(git.BranchPrefix+pr.BaseBranch, pr.GetGitRefName())
+
+	prInfo, err := repo.GetCompareInfo(repo.Path, git.BranchPrefix+pr.BaseBranch, pr.GetGitRefName(), false, false)
+	if err != nil {
+		return err
+	}
+	// Use the merge base as the base instead of the main branch to avoid problems
+	// if the pull request is out of date with the base branch.
+	changedFiles, err := repo.GetFilesChangedBetween(prInfo.MergeBase, pr.HeadCommitID)
 	if err != nil {
 		return err
 	}
diff --git a/tests/integration/codeowner_test.go b/tests/integration/codeowner_test.go
new file mode 100644
index 000000000..e1324782f
--- /dev/null
+++ b/tests/integration/codeowner_test.go
@@ -0,0 +1,118 @@
+// Copyright 2024 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package integration
+
+import (
+	"context"
+	"fmt"
+	"net/url"
+	"os"
+	"path"
+	"strings"
+	"testing"
+	"time"
+
+	issues_model "code.gitea.io/gitea/models/issues"
+	unit_model "code.gitea.io/gitea/models/unit"
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/git"
+	files_service "code.gitea.io/gitea/services/repository/files"
+	"code.gitea.io/gitea/tests"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCodeOwner(t *testing.T) {
+	onGiteaRun(t, func(t *testing.T, u *url.URL) {
+		user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+
+		// Create the repo.
+		repo, _, f := CreateDeclarativeRepo(t, user2, "",
+			[]unit_model.Type{unit_model.TypePullRequests}, nil,
+			[]*files_service.ChangeRepoFile{
+				{
+					Operation:     "create",
+					TreePath:      "CODEOWNERS",
+					ContentReader: strings.NewReader("README.md @user5\ntest-file @user4"),
+				},
+			},
+		)
+		defer f()
+
+		dstPath := t.TempDir()
+		r := fmt.Sprintf("%suser2/%s.git", u.String(), repo.Name)
+		u, _ = url.Parse(r)
+		u.User = url.UserPassword("user2", userPassword)
+		assert.NoError(t, git.CloneWithArgs(context.Background(), nil, u.String(), dstPath, git.CloneRepoOptions{}))
+
+		t.Run("Normal", func(t *testing.T) {
+			defer tests.PrintCurrentTest(t)()
+
+			err := os.WriteFile(path.Join(dstPath, "README.md"), []byte("## test content"), 0o666)
+			assert.NoError(t, err)
+
+			err = git.AddChanges(dstPath, true)
+			assert.NoError(t, err)
+
+			err = git.CommitChanges(dstPath, git.CommitChangesOptions{
+				Committer: &git.Signature{
+					Email: "user2@example.com",
+					Name:  "user2",
+					When:  time.Now(),
+				},
+				Author: &git.Signature{
+					Email: "user2@example.com",
+					Name:  "user2",
+					When:  time.Now(),
+				},
+				Message: "Add README.",
+			})
+			assert.NoError(t, err)
+
+			err = git.NewCommand(git.DefaultContext, "push", "origin", "HEAD:refs/for/main", "-o", "topic=codeowner-normal").Run(&git.RunOpts{Dir: dstPath})
+			assert.NoError(t, err)
+
+			pr := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{BaseRepoID: repo.ID, HeadBranch: "user2/codeowner-normal"})
+			unittest.AssertExistsIf(t, true, &issues_model.Review{IssueID: pr.IssueID, Type: issues_model.ReviewTypeRequest, ReviewerID: 5})
+		})
+
+		t.Run("Out of date", func(t *testing.T) {
+			defer tests.PrintCurrentTest(t)()
+
+			// Push the changes made from the previous subtest.
+			assert.NoError(t, git.NewCommand(git.DefaultContext, "push", "origin").Run(&git.RunOpts{Dir: dstPath}))
+
+			// Reset the tree to the previous commit.
+			assert.NoError(t, git.NewCommand(git.DefaultContext, "reset", "--hard", "HEAD~1").Run(&git.RunOpts{Dir: dstPath}))
+
+			err := os.WriteFile(path.Join(dstPath, "test-file"), []byte("## test content"), 0o666)
+			assert.NoError(t, err)
+
+			err = git.AddChanges(dstPath, true)
+			assert.NoError(t, err)
+
+			err = git.CommitChanges(dstPath, git.CommitChangesOptions{
+				Committer: &git.Signature{
+					Email: "user2@example.com",
+					Name:  "user2",
+					When:  time.Now(),
+				},
+				Author: &git.Signature{
+					Email: "user2@example.com",
+					Name:  "user2",
+					When:  time.Now(),
+				},
+				Message: "Add test-file.",
+			})
+			assert.NoError(t, err)
+
+			err = git.NewCommand(git.DefaultContext, "push", "origin", "HEAD:refs/for/main", "-o", "topic=codeowner-out-of-date").Run(&git.RunOpts{Dir: dstPath})
+			assert.NoError(t, err)
+
+			pr := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{BaseRepoID: repo.ID, HeadBranch: "user2/codeowner-out-of-date"})
+			unittest.AssertExistsIf(t, true, &issues_model.Review{IssueID: pr.IssueID, Type: issues_model.ReviewTypeRequest, ReviewerID: 4})
+			unittest.AssertExistsIf(t, false, &issues_model.Review{IssueID: pr.IssueID, Type: issues_model.ReviewTypeRequest, ReviewerID: 5})
+		})
+	})
+}