diff --git a/builtin/show-index.c b/builtin/show-index.c
index 3152d3c74b..756d632b51 100644
--- a/builtin/show-index.c
+++ b/builtin/show-index.c
@@ -40,6 +40,15 @@ int cmd_show_index(int argc,
 		repo_set_hash_algo(the_repository, hash_algo);
 	}
 
+	/*
+	 * Fallback to SHA1 if we are running outside of a repository.
+	 *
+	 * TODO: Figure out and implement a way to detect the hash algorithm in use by the
+	 *       the index file passed in and use that instead.
+	 */
+	if (!the_hash_algo)
+		repo_set_hash_algo(the_repository, GIT_HASH_SHA1);
+
 	hashsz = the_hash_algo->rawsz;
 
 	if (fread(top_index, 2 * 4, 1, stdin) != 1)
diff --git a/t/t5300-pack-object.sh b/t/t5300-pack-object.sh
index 53dc3cbf90..d1d6248558 100755
--- a/t/t5300-pack-object.sh
+++ b/t/t5300-pack-object.sh
@@ -525,6 +525,24 @@ test_expect_success 'index-pack --strict <pack> works in non-repo' '
 	test_path_is_file foo.idx
 '
 
+test_expect_success SHA1 'show-index works OK outside a repository' '
+	nongit git show-index <foo.idx
+'
+
+for hash in sha1 sha256
+do
+	test_expect_success 'show-index works OK outside a repository with hash algo passed in via --object-format' '
+		test_when_finished "rm -rf explicit-hash-$hash" &&
+		git init --object-format=$hash explicit-hash-$hash &&
+		test_commit -C explicit-hash-$hash one &&
+		git -C explicit-hash-$hash rev-parse one >in &&
+		git -C explicit-hash-$hash pack-objects explicit-hash-$hash <in &&
+		idx=$(echo explicit-hash-$hash/explicit-hash-$hash*.idx) &&
+		nongit git show-index --object-format=$hash <"$idx" >actual &&
+		test_line_count = 1 actual
+	'
+done
+
 test_expect_success !PTHREADS,!FAIL_PREREQS \
 	'index-pack --threads=N or pack.threads=N warns when no pthreads' '
 	test_must_fail git index-pack --threads=2 2>err &&