diff --git a/builtin/grep.c b/builtin/grep.c
index e6bcdf860cc96a..86008514869b3a 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -96,6 +96,8 @@ static pthread_cond_t cond_result;
 
 static int skip_first_line;
 
+static int grep_sparse = 0;
+
 static void add_work(struct grep_opt *opt, struct grep_source *gs)
 {
 	if (opt->binary != GREP_BINARY_TEXT)
@@ -517,15 +519,23 @@ static int grep_cache(struct grep_opt *opt,
 		strbuf_addstr(&name, repo->submodule_prefix);
 	}
 
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
 	if (repo_read_index(repo) < 0)
 		die(_("index file corrupt"));
 
-	/* TODO: audit for interaction with sparse-index. */
-	ensure_full_index(repo->index);
+	if (grep_sparse)
+		ensure_full_index(repo->index);
+
 	for (nr = 0; nr < repo->index->cache_nr; nr++) {
 		const struct cache_entry *ce = repo->index->cache[nr];
 
-		if (!cached && ce_skip_worktree(ce))
+		/*
+		 * If ce is a SKIP_WORKTREE entry, look into it when both
+		 * --sparse and --cached are given.
+		 */
+		if (!(grep_sparse && cached) && ce_skip_worktree(ce))
 			continue;
 
 		strbuf_setlen(&name, name_base_len);
@@ -963,6 +973,8 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			   PARSE_OPT_NOCOMPLETE),
 		OPT_INTEGER('m', "max-count", &opt.max_count,
 			N_("maximum number of results per file")),
+		OPT_BOOL(0, "sparse", &grep_sparse,
+			 N_("search sparse contents and expand sparse index")),
 		OPT_END()
 	};
 	grep_prefix = prefix;
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index 763c6cc684687a..795a74588d0048 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -1853,4 +1853,21 @@ test_expect_success 'mv directory from out-of-cone to in-cone' '
 	grep -e "H deep/0/1" actual
 '
 
+test_expect_success 'grep expands index using --sparse' '
+	init_repos &&
+
+	# With --sparse and --cached, do not ignore sparse entries and
+	# expand the index.
+	test_all_match git grep --sparse --cached a
+'
+
+test_expect_success 'grep is not expanded' '
+	init_repos &&
+
+	ensure_not_expanded grep a &&
+	ensure_not_expanded grep a -- deep/* &&
+	# grep does not match anything per se, so ! is used
+	ensure_not_expanded ! grep a -- folder1/*
+'
+
 test_done
diff --git a/t/t7817-grep-sparse-checkout.sh b/t/t7817-grep-sparse-checkout.sh
index eb595645657fad..ca71f526eb3193 100755
--- a/t/t7817-grep-sparse-checkout.sh
+++ b/t/t7817-grep-sparse-checkout.sh
@@ -118,13 +118,13 @@ test_expect_success 'grep searches unmerged file despite not matching sparsity p
 	test_cmp expect actual
 '
 
-test_expect_success 'grep --cached searches entries with the SKIP_WORKTREE bit' '
+test_expect_success 'grep --cached and --sparse searches entries with the SKIP_WORKTREE bit' '
 	cat >expect <<-EOF &&
 	a:text
 	b:text
 	dir/c:text
 	EOF
-	git grep --cached "text" >actual &&
+	git grep --cached --sparse "text" >actual &&
 	test_cmp expect actual
 '
 
@@ -143,7 +143,7 @@ test_expect_success 'grep --recurse-submodules honors sparse checkout in submodu
 	test_cmp expect actual
 '
 
-test_expect_success 'grep --recurse-submodules --cached searches entries with the SKIP_WORKTREE bit' '
+test_expect_success 'grep --recurse-submodules --cached and --sparse searches entries with the SKIP_WORKTREE bit' '
 	cat >expect <<-EOF &&
 	a:text
 	b:text
@@ -152,7 +152,7 @@ test_expect_success 'grep --recurse-submodules --cached searches entries with th
 	sub/B/b:text
 	sub2/a:text
 	EOF
-	git grep --recurse-submodules --cached "text" >actual &&
+	git grep --recurse-submodules --cached --sparse "text" >actual &&
 	test_cmp expect actual
 '
 
@@ -166,7 +166,7 @@ test_expect_success 'working tree grep does not search the index with CE_VALID a
 	test_cmp expect actual
 '
 
-test_expect_success 'grep --cached searches index entries with both CE_VALID and SKIP_WORKTREE' '
+test_expect_success 'grep --cached and --sparse searches index entries with both CE_VALID and SKIP_WORKTREE' '
 	cat >expect <<-EOF &&
 	a:text
 	b:text
@@ -174,7 +174,7 @@ test_expect_success 'grep --cached searches index entries with both CE_VALID and
 	EOF
 	test_when_finished "git update-index --no-assume-unchanged b" &&
 	git update-index --assume-unchanged b &&
-	git grep --cached text >actual &&
+	git grep --cached --sparse text >actual &&
 	test_cmp expect actual
 '