diff --git a/modules/gitrepo/compare_test.go b/modules/gitrepo/compare_test.go
index 2d2af0934d..3f7cc4f6a7 100644
--- a/modules/gitrepo/compare_test.go
+++ b/modules/gitrepo/compare_test.go
@@ -4,9 +4,18 @@
 package gitrepo
 
 import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"strings"
 	"testing"
+	"time"
+
+	"code.gitea.io/gitea/modules/git/gitcmd"
+	"code.gitea.io/gitea/modules/util"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 type mockRepository struct {
@@ -17,6 +26,61 @@ func (r *mockRepository) RelativePath() string {
 	return r.path
 }
 
+func commitRootTree(t *testing.T, repoDir, fileName, content, message string) string {
+	t.Helper()
+
+	require.NoError(t, gitcmd.NewCommand("read-tree", "--empty").WithDir(repoDir).Run(t.Context()))
+
+	stdout, _, err := gitcmd.NewCommand("hash-object", "-w", "--stdin").
+		WithDir(repoDir).
+		WithStdinBytes([]byte(content)).
+		RunStdString(t.Context())
+	require.NoError(t, err)
+	blobSHA := strings.TrimSpace(stdout)
+
+	_, _, err = gitcmd.NewCommand("update-index", "--add", "--replace", "--cacheinfo").
+		AddDynamicArguments("100644", blobSHA, fileName).
+		WithDir(repoDir).
+		RunStdString(t.Context())
+	require.NoError(t, err)
+
+	stdout, _, err = gitcmd.NewCommand("write-tree").WithDir(repoDir).RunStdString(t.Context())
+	require.NoError(t, err)
+	treeSHA := strings.TrimSpace(stdout)
+
+	commitTimeStr := time.Now().Format(time.RFC3339)
+	env := append(os.Environ(),
+		"GIT_AUTHOR_NAME=Test",
+		"GIT_AUTHOR_EMAIL=test@example.com",
+		"GIT_AUTHOR_DATE="+commitTimeStr,
+		"GIT_COMMITTER_NAME=Test",
+		"GIT_COMMITTER_EMAIL=test@example.com",
+		"GIT_COMMITTER_DATE="+commitTimeStr,
+	)
+
+	messageBytes := bytes.NewBufferString(message + "\n")
+	stdout, _, err = gitcmd.NewCommand("commit-tree").AddDynamicArguments(treeSHA).
+		WithEnv(env).
+		WithDir(repoDir).
+		WithStdinBytes(messageBytes.Bytes()).
+		RunStdString(t.Context())
+	require.NoError(t, err)
+
+	return strings.TrimSpace(stdout)
+}
+
+func TestMergeBaseNoCommonHistory(t *testing.T) {
+	repoDir := filepath.Join(t.TempDir(), "repo.git")
+	require.NoError(t, gitcmd.NewCommand("init").AddDynamicArguments(repoDir).Run(t.Context()))
+
+	baseCommit := commitRootTree(t, repoDir, "base.txt", "base", "base")
+	headCommit := commitRootTree(t, repoDir, "head.txt", "head", "head")
+
+	mergeBase, err := MergeBase(t.Context(), &mockRepository{path: repoDir}, baseCommit, headCommit)
+	assert.Empty(t, mergeBase)
+	assert.ErrorIs(t, err, util.ErrNotExist)
+}
+
 func TestRepoGetDivergingCommits(t *testing.T) {
 	repo := &mockRepository{path: "repo1_bare"}
 	do, err := GetDivergingCommits(t.Context(), repo, "master", "branch2")
diff --git a/modules/gitrepo/merge.go b/modules/gitrepo/merge.go
index 8d58e21c8d..92803da6fd 100644
--- a/modules/gitrepo/merge.go
+++ b/modules/gitrepo/merge.go
@@ -9,6 +9,7 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/modules/git/gitcmd"
+	"code.gitea.io/gitea/modules/util"
 )
 
 // MergeBase checks and returns merge base of two commits.
@@ -16,6 +17,9 @@ func MergeBase(ctx context.Context, repo Repository, baseCommitID, headCommitID
 	mergeBase, _, err := RunCmdString(ctx, repo, gitcmd.NewCommand("merge-base").
 		AddDashesAndList(baseCommitID, headCommitID))
 	if err != nil {
+		if gitcmd.IsErrorExitCode(err, 1) {
+			return "", util.NewNotExistErrorf("get merge-base of %s and %s failed", baseCommitID, headCommitID)
+		}
 		return "", fmt.Errorf("get merge-base of %s and %s failed: %w", baseCommitID, headCommitID, err)
 	}
 	return strings.TrimSpace(mergeBase), nil
diff --git a/options/locale/locale_en-US.json b/options/locale/locale_en-US.json
index d6c733b50b..0d31e045c8 100644
--- a/options/locale/locale_en-US.json
+++ b/options/locale/locale_en-US.json
@@ -1781,6 +1781,7 @@
   "repo.pulls.review_only_possible_for_full_diff": "Review is only possible when viewing the full diff",
   "repo.pulls.filter_changes_by_commit": "Filter by commit",
   "repo.pulls.nothing_to_compare": "These branches are equal. There is no need to create a pull request.",
+  "repo.pulls.no_common_history": "These branches do not share a common merge base. Select a different base or compare branch.",
   "repo.pulls.nothing_to_compare_have_tag": "The selected branches/tags are equal.",
   "repo.pulls.nothing_to_compare_and_allow_empty_pr": "These branches are equal. This PR will be empty.",
   "repo.pulls.has_pull_request": "A pull request between these branches already exists: <a href=\"%[1]s\">%[2]s#%[3]d</a>",
diff --git a/routers/web/repo/compare.go b/routers/web/repo/compare.go
index 733f7346f6..3f004cee88 100644
--- a/routers/web/repo/compare.go
+++ b/routers/web/repo/compare.go
@@ -413,6 +413,10 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 
 	compareInfo, err := git_service.GetCompareInfo(ctx, baseRepo, headRepo, headGitRepo, baseRef, headRef, compareReq.DirectComparison(), fileOnly)
 	if err != nil {
+		if errors.Is(err, util.ErrNotExist) {
+			ctx.Data["IsNoMergeBase"] = true
+			return compareInfo
+		}
 		ctx.ServerError("GetCompareInfo", err)
 		return nil
 	}
@@ -603,9 +607,18 @@ func CompareDiff(ctx *context.Context) {
 	ctx.Data["PullRequestWorkInProgressPrefixes"] = setting.Repository.PullRequest.WorkInProgressPrefixes
 	ctx.Data["CompareInfo"] = ci
 
-	nothingToCompare := PrepareCompareDiff(ctx, ci, gitdiff.GetWhitespaceFlag(ctx.Data["WhitespaceBehavior"].(string)))
-	if ctx.Written() {
-		return
+	var nothingToCompare bool
+	noMergeBase := ctx.Data["IsNoMergeBase"] == true
+	if noMergeBase {
+		ctx.Flash.Error(ctx.Tr("repo.pulls.no_common_history"), true)
+		ctx.Data["PageIsComparePull"] = false
+		ctx.Data["CommitCount"] = 0
+		nothingToCompare = true
+	} else {
+		nothingToCompare = PrepareCompareDiff(ctx, ci, gitdiff.GetWhitespaceFlag(ctx.Data["WhitespaceBehavior"].(string)))
+		if ctx.Written() {
+			return
+		}
 	}
 
 	baseTags, err := repo_model.GetTagNamesByRepoID(ctx, ctx.Repo.Repository.ID)
@@ -621,16 +634,13 @@ func CompareDiff(ctx *context.Context) {
 		return
 	}
 
-	headBranches, err := git_model.FindBranchNames(ctx, git_model.FindBranchOptions{
-		RepoID:          ci.HeadRepo.ID,
-		ListOptions:     db.ListOptionsAll,
-		IsDeletedBranch: optional.Some(false),
-	})
+	headBranches, headTags, err := getBranchesAndTagsForRepo(ctx, ci.HeadRepo)
 	if err != nil {
-		ctx.ServerError("GetBranches", err)
+		ctx.ServerError("GetBranchesAndTagsForRepo", err)
 		return
 	}
 	ctx.Data["HeadBranches"] = headBranches
+	ctx.Data["HeadTags"] = headTags
 
 	// For compare repo branches
 	PrepareBranchList(ctx)
@@ -638,12 +648,10 @@ func CompareDiff(ctx *context.Context) {
 		return
 	}
 
-	headTags, err := repo_model.GetTagNamesByRepoID(ctx, ci.HeadRepo.ID)
-	if err != nil {
-		ctx.ServerError("GetTagNamesByRepoID", err)
+	if noMergeBase {
+		ctx.HTML(http.StatusOK, tplCompare)
 		return
 	}
-	ctx.Data["HeadTags"] = headTags
 
 	if ctx.Data["PageIsComparePull"] == true {
 		pr, err := issues_model.GetUnmergedPullRequest(ctx, ci.HeadRepo.ID, ctx.Repo.Repository.ID, ci.HeadRef.ShortName(), ci.BaseRef.ShortName(), issues_model.PullRequestFlowGithub)
diff --git a/routers/web/repo/pull.go b/routers/web/repo/pull.go
index 3933da8764..aa3e6b02b1 100644
--- a/routers/web/repo/pull.go
+++ b/routers/web/repo/pull.go
@@ -1366,6 +1366,10 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 	if ctx.Written() {
 		return
 	}
+	if ctx.Data["IsNoMergeBase"] == true {
+		ctx.JSONError(ctx.Tr("repo.pulls.no_common_history"))
+		return
+	}
 
 	validateRet := ValidateRepoMetasForNewIssue(ctx, *form, true)
 	if ctx.Written() {
diff --git a/services/git/compare.go b/services/git/compare.go
index 251a035058..71bacfb6bd 100644
--- a/services/git/compare.go
+++ b/services/git/compare.go
@@ -76,7 +76,7 @@ func GetCompareInfo(ctx context.Context, baseRepo, headRepo *repo_model.Reposito
 	if !directComparison {
 		compareInfo.MergeBase, err = gitrepo.MergeBase(ctx, headRepo, compareInfo.BaseCommitID, compareInfo.HeadCommitID)
 		if err != nil {
-			return nil, fmt.Errorf("MergeBase: %w", err)
+			return compareInfo, fmt.Errorf("MergeBase: %w", err)
 		}
 	} else {
 		compareInfo.MergeBase = compareInfo.BaseCommitID
diff --git a/templates/repo/diff/compare.tmpl b/templates/repo/diff/compare.tmpl
index afd44f26a4..9ed4b73174 100644
--- a/templates/repo/diff/compare.tmpl
+++ b/templates/repo/diff/compare.tmpl
@@ -13,6 +13,7 @@
 				{{ctx.Locale.Tr "action.compare_commits_general"}}
 			{{end}}
 		</h2>
+		{{template "base/alert" .}}
 		{{$BaseCompareName := $.Repository.FullName -}}
 		{{$HeadCompareName := $.HeadRepo.FullName -}}
 		{{$OwnForkCompareName := "" -}}
diff --git a/tests/integration/compare_test.go b/tests/integration/compare_test.go
index a3cb538d5b..f926f10696 100644
--- a/tests/integration/compare_test.go
+++ b/tests/integration/compare_test.go
@@ -4,19 +4,25 @@
 package integration
 
 import (
+	"bytes"
 	"fmt"
 	"net/http"
 	"net/url"
+	"os"
 	"strings"
 	"testing"
+	"time"
 
+	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/git/gitcmd"
 	"code.gitea.io/gitea/modules/test"
 	repo_service "code.gitea.io/gitea/services/repository"
 	"code.gitea.io/gitea/tests"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestCompareTag(t *testing.T) {
@@ -124,6 +130,76 @@ func TestCompareBranches(t *testing.T) {
 	inspectCompare(t, htmlDoc, diffCount, diffChanges)
 }
 
+func createUnrelatedBranch(t *testing.T, repo *repo_model.Repository, user *user_model.User, branchName string) {
+	t.Helper()
+
+	repoPath := repo_model.RepoPath(user.Name, repo.Name)
+	require.NoError(t, gitcmd.NewCommand("read-tree", "--empty").WithDir(repoPath).Run(t.Context()))
+
+	stdout, _, err := gitcmd.NewCommand("hash-object", "-w", "--stdin").
+		WithDir(repoPath).
+		WithStdinBytes([]byte("Unrelated File")).
+		RunStdString(t.Context())
+	require.NoError(t, err)
+	blobSHA := strings.TrimSpace(stdout)
+
+	_, _, err = gitcmd.NewCommand("update-index", "--add", "--replace", "--cacheinfo").
+		AddDynamicArguments("100644", blobSHA, "unrelated.txt").
+		WithDir(repoPath).
+		RunStdString(t.Context())
+	require.NoError(t, err)
+
+	stdout, _, err = gitcmd.NewCommand("write-tree").WithDir(repoPath).RunStdString(t.Context())
+	require.NoError(t, err)
+	treeSHA := strings.TrimSpace(stdout)
+
+	commitTimeStr := time.Now().Format(time.RFC3339)
+	doerSig := user.NewGitSig()
+	env := append(os.Environ(),
+		"GIT_AUTHOR_NAME="+doerSig.Name,
+		"GIT_AUTHOR_EMAIL="+doerSig.Email,
+		"GIT_AUTHOR_DATE="+commitTimeStr,
+		"GIT_COMMITTER_NAME="+doerSig.Name,
+		"GIT_COMMITTER_EMAIL="+doerSig.Email,
+		"GIT_COMMITTER_DATE="+commitTimeStr,
+	)
+
+	messageBytes := bytes.NewBufferString("Unrelated\n")
+	stdout, _, err = gitcmd.NewCommand("commit-tree").AddDynamicArguments(treeSHA).
+		WithEnv(env).
+		WithDir(repoPath).
+		WithStdinBytes(messageBytes.Bytes()).
+		RunStdString(t.Context())
+	require.NoError(t, err)
+	commitSHA := strings.TrimSpace(stdout)
+
+	_, _, err = gitcmd.NewCommand("branch").AddDynamicArguments(branchName, commitSHA).
+		WithDir(repoPath).
+		RunStdString(t.Context())
+	require.NoError(t, err)
+}
+
+func TestCompareBranchesNoCommonMergeBase(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{Name: "user2"})
+	repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{OwnerID: user2.ID, Name: "repo1"})
+	createUnrelatedBranch(t, repo1, user2, "unrelated-history")
+
+	session := loginUser(t, "user2")
+	req := NewRequest(t, "GET", "/user2/repo1/compare/master...unrelated-history")
+	resp := session.MakeRequest(t, req, http.StatusOK)
+	body := resp.Body.String()
+	htmlDoc := NewHTMLParser(t, resp.Body)
+
+	selection := htmlDoc.doc.Find(".ui.dropdown.select-branch")
+	assert.Lenf(t, selection.Nodes, 2, "The template has changed")
+	assert.Contains(t, body, "These branches do not share a common merge base")
+	assert.Equal(t, 1, htmlDoc.doc.Find(`a.item[href="/user2/repo1/compare/master...unrelated-history"]`).Length())
+	assert.Equal(t, 1, htmlDoc.doc.Find(`a.item[href="/user2/repo1/compare/master...master"]`).Length())
+	assert.Equal(t, 0, htmlDoc.doc.Find(".pullrequest-form").Length())
+}
+
 func TestCompareCodeExpand(t *testing.T) {
 	onGiteaRun(t, func(t *testing.T, u *url.URL) {
 		user1 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 1})