diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index 1f7d7c07d..309a143f5 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -615,15 +615,42 @@ struct GitInputScheme : InputScheme
             AutoDelete delTmpGitDir(tmpGitDir, true);
 
             runProgram("git", true, { "-c", "init.defaultBranch=" + gitInitialBranch, "init", tmpDir, "--separate-git-dir", tmpGitDir });
-            // TODO: repoDir might lack the ref (it only checks if rev
-            // exists, see FIXME above) so use a big hammer and fetch
-            // everything to ensure we get the rev.
-            runProgram("git", true, { "-C", tmpDir, "fetch", "--quiet", "--force",
-                                      "--update-head-ok", "--", repoDir, "refs/*:refs/*" });
+
+            {
+                // TODO: repoDir might lack the ref (it only checks if rev
+                // exists, see FIXME above) so use a big hammer and fetch
+                // everything to ensure we get the rev.
+                Activity act(*logger, lvlTalkative, actUnknown, fmt("making temporary clone of '%s'", repoDir));
+                runProgram("git", true, { "-C", tmpDir, "fetch", "--quiet", "--force",
+                        "--update-head-ok", "--", repoDir, "refs/*:refs/*" });
+            }
 
             runProgram("git", true, { "-C", tmpDir, "checkout", "--quiet", input.getRev()->gitRev() });
-            runProgram("git", true, { "-C", tmpDir, "remote", "add", "origin", actualUrl });
-            runProgram("git", true, { "-C", tmpDir, "submodule", "--quiet", "update", "--init", "--recursive" });
+
+            /* Ensure that we use the correct origin for fetching
+               submodules. This matters for submodules with relative
+               URLs. */
+            if (isLocal) {
+                writeFile(tmpGitDir + "/config", readFile(repoDir + "/" + gitDir + "/config"));
+
+                /* Restore the config.bare setting we may have just
+                   copied erroneously from the user's repo. */
+                runProgram("git", true, { "-C", tmpDir, "config", "core.bare", "false" });
+            } else
+                runProgram("git", true, { "-C", tmpDir, "config", "remote.origin.url", actualUrl });
+
+            /* As an optimisation, copy the modules directory of the
+               source repo if it exists. */
+            auto modulesPath = repoDir + "/" + gitDir + "/modules";
+            if (pathExists(modulesPath)) {
+                Activity act(*logger, lvlTalkative, actUnknown, fmt("copying submodules of '%s'", actualUrl));
+                runProgram("cp", true, { "-R", "--", modulesPath, tmpGitDir + "/modules" });
+            }
+
+            {
+                Activity act(*logger, lvlTalkative, actUnknown, fmt("fetching submodules of '%s'", actualUrl));
+                runProgram("git", true, { "-C", tmpDir, "submodule", "--quiet", "update", "--init", "--recursive" });
+            }
 
             filter = isNotDotGitDirectory;
         } else {
diff --git a/tests/fetchGitSubmodules.sh b/tests/fetchGitSubmodules.sh
index 50da4cb97..08ccaa3cd 100644
--- a/tests/fetchGitSubmodules.sh
+++ b/tests/fetchGitSubmodules.sh
@@ -104,3 +104,28 @@ noSubmoduleRepoBaseline=$(nix eval --raw --expr "(builtins.fetchGit { url = file
 noSubmoduleRepo=$(nix eval --raw --expr "(builtins.fetchGit { url = file://$subRepo; rev = \"$subRev\"; submodules = true; }).outPath")
 
 [[ $noSubmoduleRepoBaseline == $noSubmoduleRepo ]]
+
+# Test relative submodule URLs.
+rm $TEST_HOME/.cache/nix/fetcher-cache*
+rm -rf $rootRepo/.git $rootRepo/.gitmodules $rootRepo/sub
+initGitRepo $rootRepo
+git -C $rootRepo submodule add ../gitSubmodulesSub sub
+git -C $rootRepo commit -m "Add submodule"
+rev2=$(git -C $rootRepo rev-parse HEAD)
+pathWithRelative=$(nix eval --raw --expr "(builtins.fetchGit { url = file://$rootRepo; rev = \"$rev2\"; submodules = true; }).outPath")
+diff -r -x .gitmodules $pathWithSubmodules $pathWithRelative
+
+# Test clones that have an upstream with relative submodule URLs.
+rm $TEST_HOME/.cache/nix/fetcher-cache*
+cloneRepo=$TEST_ROOT/a/b/gitSubmodulesClone # NB /a/b to make the relative path not work relative to $cloneRepo
+git clone $rootRepo $cloneRepo
+pathIndirect=$(nix eval --raw --expr "(builtins.fetchGit { url = file://$cloneRepo; rev = \"$rev2\"; submodules = true; }).outPath")
+[[ $pathIndirect = $pathWithRelative ]]
+
+# Test that if the clone has the submodule already, we're not fetching
+# it again.
+git -C $cloneRepo submodule update --init
+rm $TEST_HOME/.cache/nix/fetcher-cache*
+rm -rf $subRepo
+pathSubmoduleGone=$(nix eval --raw --expr "(builtins.fetchGit { url = file://$cloneRepo; rev = \"$rev2\"; submodules = true; }).outPath")
+[[ $pathSubmoduleGone = $pathWithRelative ]]