diff --git a/src/libfetchers/tarball.cc b/src/libfetchers/tarball.cc
index b1ebd749df6..94e232fa70c 100644
--- a/src/libfetchers/tarball.cc
+++ b/src/libfetchers/tarball.cc
@@ -114,6 +114,12 @@ static DownloadTarballResult downloadTarball_(
     // it is not in fact a tarball.
     if (url.scheme == "file") {
         std::filesystem::path localPath = renderUrlPathEnsureLegal(url.path);
+        if (!localPath.is_absolute()) {
+            throw Error(
+                "tarball '%s' must use an absolute path. "
+                "The 'file' scheme does not support relative paths.",
+                url);
+        }
         if (!exists(localPath)) {
             throw Error("tarball '%s' does not exist.", localPath);
         }
diff --git a/tests/functional/lang/eval-fail-fetchTree-relative-path.err.exp b/tests/functional/lang/eval-fail-fetchTree-relative-path.err.exp
new file mode 100644
index 00000000000..863f143d19a
--- /dev/null
+++ b/tests/functional/lang/eval-fail-fetchTree-relative-path.err.exp
@@ -0,0 +1,10 @@
+error:
+       … while calling the 'fetchTree' builtin
+         at /pwd/lang/eval-fail-fetchTree-relative-path.nix:1:1:
+            1| builtins.fetchTree "file:./relative-path.tar.gz"
+             | ^
+            2|
+
+       … while fetching the input 'file:./relative-path.tar.gz'
+
+       error: tarball 'file:./relative-path.tar.gz' must use an absolute path. The 'file' scheme does not support relative paths.
diff --git a/tests/functional/lang/eval-fail-fetchTree-relative-path.nix b/tests/functional/lang/eval-fail-fetchTree-relative-path.nix
new file mode 100644
index 00000000000..38826758134
--- /dev/null
+++ b/tests/functional/lang/eval-fail-fetchTree-relative-path.nix
@@ -0,0 +1 @@
+builtins.fetchTree "file:./relative-path.tar.gz"