diff --git a/release-notes/8.0.0/feat/3615.md b/release-notes/8.0.0/feat/3615.md
new file mode 100644
index 000000000..f2dd891c9
--- /dev/null
+++ b/release-notes/8.0.0/feat/3615.md
@@ -0,0 +1,5 @@
+Support the [Nix tarball fetcher immutable link protocol](https://github.com/nixos/nix/blob/56763ff918eb308db23080e560ed2ea3e00c80a7/doc/manual/src/protocols/tarball-fetcher.md) on archive URLs, so Forgejo-generated tarballs for branches will go into Nix's `flake.lock` as their respective commit URLs and `nix flake update` will just work. This allows natively fetching Forgejo repositories for Nix flake inputs as tarballs rather than as Git repositories, significantly improving fetch times and avoiding depending on Git at runtime.
+
+Concretely, Forgejo now returns a header of the following format from its archive URLs: `Link: <https://my-forgejo/api/v1/someuser/somerepo/archive/some-commit-hash.tar.gz?rev=some-commit-hash> rel="immutable"`.
+
+Example usage: `inputs.meow.url = "https://my-forgejo/someuser/somerepo/archive/main.tar.gz";` in `flake.nix`. For a private repository, configure `netrc-file` in `nix.conf` and use `https://my-forgejo/api/v1/repos/someuser/somerepo/archive/main.tar.gz` as a URL instead, since the normal archive endpoint doesn't support tokens.
diff --git a/routers/api/v1/repo/file.go b/routers/api/v1/repo/file.go
index 70eb3fbc2..34ccc929a 100644
--- a/routers/api/v1/repo/file.go
+++ b/routers/api/v1/repo/file.go
@@ -326,6 +326,12 @@ func archiveDownload(ctx *context.APIContext) {
 func download(ctx *context.APIContext, archiveName string, archiver *repo_model.RepoArchiver) {
 	downloadName := ctx.Repo.Repository.Name + "-" + archiveName
 
+	// Add nix format link header so tarballs lock correctly:
+	// https://github.com/nixos/nix/blob/56763ff918eb308db23080e560ed2ea3e00c80a7/doc/manual/src/protocols/tarball-fetcher.md
+	ctx.Resp.Header().Add("Link", fmt.Sprintf("<%s/archive/%s.tar.gz?rev=%s>; rel=\"immutable\"",
+		ctx.Repo.Repository.APIURL(),
+		archiver.CommitID, archiver.CommitID))
+
 	rPath := archiver.RelativePath()
 	if setting.RepoArchive.Storage.MinioConfig.ServeDirect {
 		// If we have a signed url (S3, object storage), redirect to this directly.
diff --git a/routers/web/repo/repo.go b/routers/web/repo/repo.go
index a5f6c02c7..abd68630a 100644
--- a/routers/web/repo/repo.go
+++ b/routers/web/repo/repo.go
@@ -480,6 +480,12 @@ func Download(ctx *context.Context) {
 func download(ctx *context.Context, archiveName string, archiver *repo_model.RepoArchiver) {
 	downloadName := ctx.Repo.Repository.Name + "-" + archiveName
 
+	// Add nix format link header so tarballs lock correctly:
+	// https://github.com/nixos/nix/blob/56763ff918eb308db23080e560ed2ea3e00c80a7/doc/manual/src/protocols/tarball-fetcher.md
+	ctx.Resp.Header().Add("Link", fmt.Sprintf("<%s/archive/%s.tar.gz?rev=%s>; rel=\"immutable\"",
+		ctx.Repo.Repository.APIURL(),
+		archiver.CommitID, archiver.CommitID))
+
 	rPath := archiver.RelativePath()
 	if setting.RepoArchive.Storage.MinioConfig.ServeDirect {
 		// If we have a signed url (S3, object storage), redirect to this directly.
diff --git a/tests/integration/api_repo_archive_test.go b/tests/integration/api_repo_archive_test.go
index c574d4945..a4ae59961 100644
--- a/tests/integration/api_repo_archive_test.go
+++ b/tests/integration/api_repo_archive_test.go
@@ -8,6 +8,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"regexp"
 	"testing"
 
 	auth_model "code.gitea.io/gitea/models/auth"
@@ -41,6 +42,16 @@ func TestAPIDownloadArchive(t *testing.T) {
 	assert.Len(t, bs, 266)
 	assert.EqualValues(t, "application/gzip", resp.Header().Get("Content-Type"))
 
+	// Must return a link to a commit ID as the "immutable" archive link
+	linkHeaderRe := regexp.MustCompile(`<(?P<url>https?://.*/api/v1/repos/user2/repo1/archive/[a-f0-9]+\.tar\.gz.*)>; rel="immutable"`)
+	m := linkHeaderRe.FindStringSubmatch(resp.Header().Get("Link"))
+	assert.NotEmpty(t, m[1])
+	resp = MakeRequest(t, NewRequest(t, "GET", m[1]).AddTokenAuth(token), http.StatusOK)
+	bs2, err := io.ReadAll(resp.Body)
+	assert.NoError(t, err)
+	// The locked URL should give the same bytes as the non-locked one
+	assert.EqualValues(t, bs, bs2)
+
 	link, _ = url.Parse(fmt.Sprintf("/api/v1/repos/%s/%s/archive/master.bundle", user2.Name, repo.Name))
 	resp = MakeRequest(t, NewRequest(t, "GET", link.String()).AddTokenAuth(token), http.StatusOK)
 	bs, err = io.ReadAll(resp.Body)