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 ]]