diff --git a/Documentation/config/advice.txt b/Documentation/config/advice.txt
index fa61241756..0ba8989820 100644
--- a/Documentation/config/advice.txt
+++ b/Documentation/config/advice.txt
@@ -116,6 +116,10 @@ advice.*::
 	skippedCherryPicks::
 		Shown when linkgit:git-rebase[1] skips a commit that has already
 		been cherry-picked onto the upstream branch.
+	sparseIndexExpanded::
+		Shown when a sparse index is expanded to a full index, which is likely
+		due to an unexpected set of files existing outside of the
+		sparse-checkout.
 	statusAheadBehind::
 		Shown when linkgit:git-status[1] computes the ahead/behind
 		counts for a local ref compared to its remote tracking ref,
diff --git a/advice.c b/advice.c
index 558a46fc0b..6b879d805c 100644
--- a/advice.c
+++ b/advice.c
@@ -78,6 +78,7 @@ static struct {
 	[ADVICE_SEQUENCER_IN_USE]			= { "sequencerInUse" },
 	[ADVICE_SET_UPSTREAM_FAILURE]			= { "setUpstreamFailure" },
 	[ADVICE_SKIPPED_CHERRY_PICKS]			= { "skippedCherryPicks" },
+	[ADVICE_SPARSE_INDEX_EXPANDED]			= { "sparseIndexExpanded" },
 	[ADVICE_STATUS_AHEAD_BEHIND_WARNING]		= { "statusAheadBehindWarning" },
 	[ADVICE_STATUS_HINTS]				= { "statusHints" },
 	[ADVICE_STATUS_U_OPTION]			= { "statusUoption" },
diff --git a/advice.h b/advice.h
index 5105d90129..d7466bc0ef 100644
--- a/advice.h
+++ b/advice.h
@@ -45,6 +45,7 @@ enum advice_type {
 	ADVICE_SEQUENCER_IN_USE,
 	ADVICE_SET_UPSTREAM_FAILURE,
 	ADVICE_SKIPPED_CHERRY_PICKS,
+	ADVICE_SPARSE_INDEX_EXPANDED,
 	ADVICE_STATUS_AHEAD_BEHIND_WARNING,
 	ADVICE_STATUS_HINTS,
 	ADVICE_STATUS_U_OPTION,
diff --git a/sparse-index.c b/sparse-index.c
index 9913a6078c..9958656ded 100644
--- a/sparse-index.c
+++ b/sparse-index.c
@@ -12,6 +12,22 @@
 #include "config.h"
 #include "dir.h"
 #include "fsmonitor-ll.h"
+#include "advice.h"
+
+/**
+ * This global is used by expand_index() to determine if we should give the
+ * advice for advice.sparseIndexExpanded when expanding a sparse index to a full
+ * one. However, this is sometimes done on purpose, such as in the sparse-checkout
+ * builtin, even when index.sparse=false. This may be disabled in
+ * convert_to_sparse().
+ */
+static int give_advice_on_expansion = 1;
+#define ADVICE_MSG \
+	"The sparse index is expanding to a full index, a slow operation.\n"   \
+	"Your working directory likely has contents that are outside of\n"     \
+	"your sparse-checkout patterns. Use 'git sparse-checkout list' to\n"   \
+	"see your sparse-checkout definition and compare it to your working\n" \
+	"directory contents. Running 'git clean' may assist in this cleanup."
 
 struct modify_index_context {
 	struct index_state *write;
@@ -183,6 +199,12 @@ int convert_to_sparse(struct index_state *istate, int flags)
 	    !is_sparse_index_allowed(istate, flags))
 		return 0;
 
+	/*
+	 * If we are purposefully collapsing a full index, then don't give
+	 * advice when it is expanded later.
+	 */
+	give_advice_on_expansion = 0;
+
 	/*
 	 * NEEDSWORK: If we have unmerged entries, then stay full.
 	 * Unmerged entries prevent the cache-tree extension from working.
@@ -328,6 +350,12 @@ void expand_index(struct index_state *istate, struct pattern_list *pl)
 			pl = NULL;
 	}
 
+	if (!pl && give_advice_on_expansion) {
+		give_advice_on_expansion = 0;
+		advise_if_enabled(ADVICE_SPARSE_INDEX_EXPANDED,
+				  _(ADVICE_MSG));
+	}
+
 	/*
 	 * A NULL pattern set indicates we are expanding a full index, so
 	 * we use a special region name that indicates the full expansion.
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index 2f1ae5fd3b..a2c0e1b4dc 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -159,7 +159,10 @@ init_repos () {
 	git -C sparse-checkout sparse-checkout set deep &&
 	git -C sparse-index sparse-checkout init --cone --sparse-index &&
 	test_cmp_config -C sparse-index true index.sparse &&
-	git -C sparse-index sparse-checkout set deep
+	git -C sparse-index sparse-checkout set deep &&
+
+	# Disable this message to keep stderr the same.
+	git -C sparse-index config advice.sparseIndexExpanded false
 }
 
 init_repos_as_submodules () {
@@ -2331,4 +2334,15 @@ test_expect_success 'sparse-index is not expanded: check-attr' '
 	ensure_not_expanded check-attr -a --cached -- folder1/a
 '
 
+test_expect_success 'advice.sparseIndexExpanded' '
+	init_repos &&
+
+	git -C sparse-index config --unset advice.sparseIndexExpanded &&
+	git -C sparse-index sparse-checkout set deep/deeper1 &&
+	mkdir -p sparse-index/deep/deeper2/deepest &&
+	touch sparse-index/deep/deeper2/deepest/bogus &&
+	git -C sparse-index status 2>err &&
+	grep "The sparse index is expanding to a full index" err
+'
+
 test_done