diff --git a/tests/custom/99_bugs/43_types_json_double_format b/tests/custom/99_bugs/43_types_json_double_format
new file mode 100644
index 00000000..9b90e373
--- /dev/null
+++ b/tests/custom/99_bugs/43_types_json_double_format
@@ -0,0 +1,24 @@
+When formatting integral double values as JSON, ensure that at least one
+decimal place is retained.
+
+-- Testcase --
+printf("%.J\n", [
+	1e100,
+	1.23,
+	4.00,
+	1.0/3*3,
+]);
+-- End --
+
+-- Args --
+-R
+-- End --
+
+-- Expect stdout --
+[
+	1e+100,
+	1.23,
+	4.0,
+	1.0
+]
+-- End --
diff --git a/types.c b/types.c
index c9472f7e..eeac8c74 100644
--- a/types.c
+++ b/types.c
@@ -1561,6 +1561,21 @@ ucv_to_stringbuf_add_padding(uc_stringbuf_t *pb, char pad_char, size_t pad_size)
 	}
 }
 
+static void
+ucv_to_stringbuf_add_double(uc_stringbuf_t *pb, double val, bool json)
+{
+	int len = ucv_stringbuf_printf(pb, "%.14g", val);
+
+	if (!json)
+		return;
+
+	for (char *p = pb->buf + pb->bpos - len; len > 0; len--, p++)
+		if (*p == '.' || *p == 'e')
+			return;
+
+	ucv_stringbuf_append(pb, ".0");
+}
+
 void
 ucv_to_stringbuf_formatted(uc_vm_t *vm, uc_stringbuf_t *pb, uc_value_t *uv, size_t depth, char pad_char, size_t pad_size)
 {
@@ -1624,7 +1639,7 @@ ucv_to_stringbuf_formatted(uc_vm_t *vm, uc_stringbuf_t *pb, uc_value_t *uv, size
 		else if (d == -INFINITY)
 			ucv_stringbuf_append(pb, "-Infinity");
 		else
-			ucv_stringbuf_printf(pb, "%.14g", d);
+			ucv_to_stringbuf_add_double(pb, d, json);
 
 		break;