diff --git a/dir.c b/dir.c
index ba4a51c296..f389eee24a 100644
--- a/dir.c
+++ b/dir.c
@@ -19,6 +19,7 @@
 #include "varint.h"
 #include "ewah/ewok.h"
 #include "fsmonitor.h"
+#include "json-writer.h"
 #include "submodule-config.h"
 
 /*
@@ -2826,7 +2827,42 @@ static void load_oid_stat(struct oid_stat *oid_stat, const unsigned char *data,
 	oid_stat->valid = 1;
 }
 
-struct untracked_cache *read_untracked_extension(const void *data, unsigned long sz)
+static void jw_object_oid_stat(struct json_writer *jw, const char *key,
+			       const struct oid_stat *oid_stat)
+{
+	jw_object_inline_begin_object(jw, key);
+	jw_object_bool(jw, "valid", oid_stat->valid);
+	jw_object_string(jw, "oid", oid_to_hex(&oid_stat->oid));
+	jw_object_stat_data(jw, "stat", &oid_stat->stat);
+	jw_end(jw);
+}
+
+static void jw_object_untracked_cache_dir(struct json_writer *jw,
+					  const struct untracked_cache_dir *ucd)
+{
+	int i;
+
+	jw_object_bool(jw, "valid", ucd->valid);
+	jw_object_bool(jw, "check-only", ucd->check_only);
+	jw_object_stat_data(jw, "stat", &ucd->stat_data);
+	jw_object_string(jw, "exclude-oid", oid_to_hex(&ucd->exclude_oid));
+	jw_object_inline_begin_array(jw, "untracked");
+	for (i = 0; i < ucd->untracked_nr; i++)
+		jw_array_string(jw, ucd->untracked[i]);
+	jw_end(jw);
+
+	jw_object_inline_begin_object(jw, "dirs");
+	for (i = 0; i < ucd->dirs_nr; i++) {
+		jw_object_inline_begin_object(jw, ucd->dirs[i]->name);
+		jw_object_untracked_cache_dir(jw, ucd->dirs[i]);
+		jw_end(jw);
+	}
+	jw_end(jw);
+}
+
+struct untracked_cache *read_untracked_extension(const void *data,
+						 unsigned long sz,
+						 struct json_writer *jw)
 {
 	struct untracked_cache *uc;
 	struct read_data rd;
@@ -2864,6 +2900,17 @@ struct untracked_cache *read_untracked_extension(const void *data, unsigned long
 	uc->dir_flags = get_be32(next + ouc_offset(dir_flags));
 	exclude_per_dir = (const char *)next + exclude_per_dir_offset;
 	uc->exclude_per_dir = xstrdup(exclude_per_dir);
+
+	if (jw) {
+		jw_object_inline_begin_object(jw, "untracked-cache");
+		jw_object_intmax(jw, "ext-size", sz);
+		jw_object_string(jw, "ident", ident);
+		jw_object_oid_stat(jw, "info/exclude", &uc->ss_info_exclude);
+		jw_object_oid_stat(jw, "excludes-file", &uc->ss_excludes_file);
+		jw_object_intmax(jw, "flags", uc->dir_flags);
+		jw_object_string(jw, "excludes-per-dir", uc->exclude_per_dir);
+	}
+
 	/* NUL after exclude_per_dir is covered by sizeof(*ouc) */
 	next += exclude_per_dir_offset + strlen(exclude_per_dir) + 1;
 	if (next >= end)
@@ -2905,6 +2952,12 @@ struct untracked_cache *read_untracked_extension(const void *data, unsigned long
 	ewah_each_bit(rd.sha1_valid, read_oid, &rd);
 	next = rd.data;
 
+	if (jw) {
+		jw_object_inline_begin_object(jw, "root");
+		jw_object_untracked_cache_dir(jw, uc->root);
+		jw_end(jw);
+	}
+
 done:
 	free(rd.ucd);
 	ewah_free(rd.valid);
@@ -2915,6 +2968,7 @@ struct untracked_cache *read_untracked_extension(const void *data, unsigned long
 		free_untracked_cache(uc);
 		uc = NULL;
 	}
+	jw_end_gently(jw);
 	return uc;
 }
 
diff --git a/dir.h b/dir.h
index 680079bbe3..80efdd05c4 100644
--- a/dir.h
+++ b/dir.h
@@ -6,6 +6,8 @@
 #include "cache.h"
 #include "strbuf.h"
 
+struct json_writer;
+
 struct dir_entry {
 	unsigned int len;
 	char name[FLEX_ARRAY]; /* more */
@@ -362,7 +364,7 @@ void untracked_cache_remove_from_index(struct index_state *, const char *);
 void untracked_cache_add_to_index(struct index_state *, const char *);
 
 void free_untracked_cache(struct untracked_cache *);
-struct untracked_cache *read_untracked_extension(const void *data, unsigned long sz);
+struct untracked_cache *read_untracked_extension(const void *data, unsigned long sz, struct json_writer *jw);
 void write_untracked_extension(struct strbuf *out, struct untracked_cache *untracked);
 void add_untracked_cache(struct index_state *istate);
 void remove_untracked_cache(struct index_state *istate);
diff --git a/json-writer.h b/json-writer.h
index 3c173647d3..f778e019a2 100644
--- a/json-writer.h
+++ b/json-writer.h
@@ -121,6 +121,12 @@ static inline void jw_object_inline_begin_array_gently(struct json_writer *jw,
 		jw_object_inline_begin_array(jw, name);
 }
 
+static inline void jw_array_inline_begin_object_gently(struct json_writer *jw)
+{
+	if (jw)
+		jw_array_inline_begin_object(jw);
+}
+
 static inline void jw_end_gently(struct json_writer *jw)
 {
 	if (jw)
diff --git a/read-cache.c b/read-cache.c
index 289705b816..d7d9ce7260 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1708,7 +1708,7 @@ static int read_index_extension(struct index_state *istate,
 			return -1;
 		break;
 	case CACHE_EXT_UNTRACKED:
-		istate->untracked = read_untracked_extension(data, sz);
+		istate->untracked = read_untracked_extension(data, sz, istate->jw);
 		break;
 	case CACHE_EXT_FSMONITOR:
 		read_fsmonitor_extension(istate, data, sz);
