diff --git a/src/libfetchers-tests/git.cc b/src/libfetchers-tests/git.cc
new file mode 100644
index 000000000..83ee1a1a2
--- /dev/null
+++ b/src/libfetchers-tests/git.cc
@@ -0,0 +1,199 @@
+#include "nix/store/store-open.hh"
+#include "nix/fetchers/fetch-settings.hh"
+#include "nix/fetchers/fetchers.hh"
+#include "nix/fetchers/git-utils.hh"
+
+#include <git2.h>
+#include <gtest/gtest.h>
+
+#include <filesystem>
+
+namespace {
+
+template<class T, void (*F)(T *)>
+struct Deleter
+{
+    void operator()(T * p) const
+    {
+        if (p)
+            F(p);
+    }
+};
+
+template<class T, void (*F)(T *)>
+using Handle = std::unique_ptr<T, Deleter<T, F>>;
+
+using RepoHandle = Handle<git_repository, git_repository_free>;
+using IndexHandle = Handle<git_index, git_index_free>;
+using TreeHandle = Handle<git_tree, git_tree_free>;
+using SigHandle = Handle<git_signature, git_signature_free>;
+using RefHandle = Handle<git_reference, git_reference_free>;
+using CommitHandle = Handle<git_commit, git_commit_free>;
+using SubmoduleHandle = Handle<git_submodule, git_submodule_free>;
+
+#define CHECK_LIBGIT(expr) ASSERT_TRUE((expr) >= 0) << git_error_last()
+
+static void commitAll(git_repository * repo, const char * msg)
+{
+    IndexHandle idx;
+    {
+        git_index * raw = nullptr;
+        CHECK_LIBGIT(git_repository_index(&raw, repo));
+        idx.reset(raw);
+    }
+    CHECK_LIBGIT(git_index_add_all(idx.get(), nullptr, 0, nullptr, nullptr));
+    CHECK_LIBGIT(git_index_write(idx.get()));
+
+    git_oid treeId{};
+    CHECK_LIBGIT(git_index_write_tree(&treeId, idx.get()));
+    TreeHandle tree;
+    {
+        git_tree * raw = nullptr;
+        CHECK_LIBGIT(git_tree_lookup(&raw, repo, &treeId));
+        tree.reset(raw);
+    }
+
+    SigHandle sig;
+    {
+        git_signature * raw = nullptr;
+        CHECK_LIBGIT(git_signature_now(&raw, "you", "you@example.com"));
+        sig.reset(raw);
+    }
+
+    git_oid commitId{};
+    if (git_repository_is_empty(repo) == 1) {
+        CHECK_LIBGIT(git_commit_create_v(&commitId, repo, "HEAD", sig.get(), sig.get(), nullptr, msg, tree.get(), 0));
+        CHECK_LIBGIT(git_reference_create(nullptr, repo, "refs/heads/main", &commitId, true, nullptr));
+        CHECK_LIBGIT(git_repository_set_head(repo, "refs/heads/main"));
+    } else {
+        RefHandle head;
+        {
+            git_reference * raw = nullptr;
+            CHECK_LIBGIT(git_repository_head(&raw, repo));
+            head.reset(raw);
+        }
+        CommitHandle parent;
+        {
+            git_commit * raw = nullptr;
+            CHECK_LIBGIT(git_commit_lookup(&raw, repo, git_reference_target(head.get())));
+            parent.reset(raw);
+        }
+        const git_commit * parents[] = {parent.get()};
+        CHECK_LIBGIT(git_commit_create(
+            &commitId,
+            repo,
+            "HEAD",
+            sig.get(),
+            sig.get(),
+            /*message_encoding=*/nullptr,
+            msg,
+            tree.get(),
+            /*parent_count=*/1,
+            &parents[0]));
+    }
+}
+
+} // namespace
+
+using namespace nix;
+
+class GitTest : public ::testing::Test
+{
+    std::unique_ptr<AutoDelete> delTmpDir;
+
+protected:
+    std::filesystem::path tmpDir;
+
+    void SetUp() override
+    {
+        tmpDir = createTempDir();
+        delTmpDir = std::make_unique<AutoDelete>(tmpDir, /*recursive=*/true);
+        nix::initLibStore(/*loadConfig=*/false);
+        git_libgit2_init();
+    }
+
+    void TearDown() override
+    {
+        delTmpDir.reset();
+    }
+};
+
+// Regression test for https://github.com/NixOS/nix/issues/13215
+TEST_F(GitTest, submodulePeriodSupport)
+{
+    auto storePath = tmpDir / "store";
+    auto repoPath = tmpDir / "repo";
+    auto submodulePath = tmpDir / "submodule";
+
+    // Set up our git directories: one top level and a submodule
+    // the submodule in the .gitmodules has the branch listed as '.'
+
+    // 1) Create sub repo
+    {
+        git_repository * raw = nullptr;
+        CHECK_LIBGIT(git_repository_init(&raw, submodulePath.string().c_str(), /*is_bare=*/0));
+        RepoHandle sub(raw);
+        writeFile(submodulePath / "lib.txt", "hello from submodule\n");
+        commitAll(sub.get(), "init sub");
+    }
+
+    // 2) Create super repo
+    RepoHandle super;
+    {
+        git_repository * raw = nullptr;
+        CHECK_LIBGIT(git_repository_init(&raw, repoPath.string().c_str(), /*is_bare=*/0));
+        super.reset(raw);
+    }
+
+    writeFile(repoPath / "README.md", "# super\n");
+    commitAll(super.get(), "init super");
+
+    // 3) Add submodule at deps/sub
+    {
+        git_repository * raw = nullptr;
+        git_clone_options cloneOpts = GIT_CLONE_OPTIONS_INIT;
+        // clone from local subPath into superPath/deps/sub
+        CHECK_LIBGIT(
+            git_clone(&raw, submodulePath.string().c_str(), (repoPath / "deps" / "sub").string().c_str(), &cloneOpts));
+        RepoHandle sub(raw);
+    }
+
+    // 4) Add submodule and set branch="."
+    SubmoduleHandle sm;
+    {
+        git_submodule * raw = nullptr;
+        CHECK_LIBGIT(git_submodule_add_setup(
+            &raw,
+            super.get(),
+            "../submodule",
+            "deps/sub",
+            /*use_gitlink=*/1));
+        sm.reset(raw);
+    }
+    CHECK_LIBGIT(git_submodule_set_branch(super.get(), git_submodule_name(sm.get()), /*branch=*/"."));
+    CHECK_LIBGIT(git_submodule_sync(sm.get()));
+
+    // 5) Finalize now that the worktree exists; libgit2 can read its HEAD OID
+    CHECK_LIBGIT(git_submodule_add_finalize(sm.get()));
+    // 6) Commit the addition in super
+    commitAll(super.get(), "Add submodule with branch='.'");
+
+    // TODO: Use dummy:// store with MemorySourceAccessor.
+    Path storeTmpDir = createTempDir();
+    auto storeTmpDirAutoDelete = AutoDelete(storeTmpDir, true);
+    ref<Store> store = openStore(storeTmpDir);
+
+    auto settings = fetchers::Settings{};
+    auto input = fetchers::Input::fromAttrs(
+        settings,
+        {
+            {"url", "file://" + repoPath.string()},
+            {"submodules", Explicit{true}},
+            {"type", "git"},
+            {"ref", "main"},
+        });
+
+    auto [accessor, i] = input.getAccessor(store);
+
+    ASSERT_EQ(accessor->readFile(CanonPath("deps/sub/lib.txt")), "hello from submodule\n");
+}
diff --git a/src/libfetchers-tests/meson.build b/src/libfetchers-tests/meson.build
index 51373aefa..cd23c1f0c 100644
--- a/src/libfetchers-tests/meson.build
+++ b/src/libfetchers-tests/meson.build
@@ -41,6 +41,7 @@ subdir('nix-meson-build-support/common')
 sources = files(
   'access-tokens.cc',
   'git-utils.cc',
+  'git.cc',
   'nix_api_fetchers.cc',
   'public-key.cc',
 )
diff --git a/src/libfetchers-tests/package.nix b/src/libfetchers-tests/package.nix
index 48c1a07d8..780618725 100644
--- a/src/libfetchers-tests/package.nix
+++ b/src/libfetchers-tests/package.nix
@@ -3,6 +3,7 @@
   buildPackages,
   stdenv,
   mkMesonExecutable,
+  writableTmpDirAsHomeHook,
 
   nix-fetchers,
   nix-fetchers-c,
@@ -57,18 +58,13 @@ mkMesonExecutable (finalAttrs: {
         runCommand "${finalAttrs.pname}-run"
           {
             meta.broken = !stdenv.hostPlatform.emulatorAvailable buildPackages;
+            buildInputs = [ writableTmpDirAsHomeHook ];
           }
-          (
-            lib.optionalString stdenv.hostPlatform.isWindows ''
-              export HOME="$PWD/home-dir"
-              mkdir -p "$HOME"
-            ''
-            + ''
-              export _NIX_TEST_UNIT_DATA=${resolvePath ./data}
-              ${stdenv.hostPlatform.emulator buildPackages} ${lib.getExe finalAttrs.finalPackage}
-              touch $out
-            ''
-          );
+          ''
+            export _NIX_TEST_UNIT_DATA=${resolvePath ./data}
+            ${stdenv.hostPlatform.emulator buildPackages} ${lib.getExe finalAttrs.finalPackage}
+            touch $out
+          '';
     };
   };
 
diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index 43105c699..bd1e1fffe 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -739,8 +739,16 @@ struct GitInputScheme : InputScheme
                 fetchers::Attrs attrs;
                 attrs.insert_or_assign("type", "git");
                 attrs.insert_or_assign("url", resolved);
-                if (submodule.branch != "")
-                    attrs.insert_or_assign("ref", submodule.branch);
+                if (submodule.branch != "") {
+                    // A special value of . is used to indicate that the name of the branch in the submodule
+                    // should be the same name as the current branch in the current repository.
+                    // https://git-scm.com/docs/gitmodules
+                    if (submodule.branch == ".") {
+                        attrs.insert_or_assign("ref", ref);
+                    } else {
+                        attrs.insert_or_assign("ref", submodule.branch);
+                    }
+                }
                 attrs.insert_or_assign("rev", submoduleRev.gitRev());
                 attrs.insert_or_assign("exportIgnore", Explicit<bool>{exportIgnore});
                 attrs.insert_or_assign("submodules", Explicit<bool>{true});