diff --git a/NEWS b/NEWS
index 4ffc2b8c0bcc..b5d863271cb0 100644
--- a/NEWS
+++ b/NEWS
@@ -57,6 +57,8 @@ PHP                                                                        NEWS
     (Ilia Alshanetsky)
   . Fixed Collator attribute and strength methods not rejecting an
     unconstructed Collator. (Ilia Alshanetsky)
+  . Fixed grapheme_strstr() and grapheme_stristr() returning incorrect substrings
+    when supplementary characters precede the match. (Weilin Du)
 
 - Lexbor:
   . Merge patches lexbor/lexbor@8a14bc0 and lexbor/lexbor@f67ce4b, fixing a
diff --git a/ext/intl/grapheme/grapheme_util.c b/ext/intl/grapheme/grapheme_util.c
index f0c79785483f..66c3876a5bd2 100644
--- a/ext/intl/grapheme/grapheme_util.c
+++ b/ext/intl/grapheme/grapheme_util.c
@@ -190,7 +190,7 @@ int32_t grapheme_strpos_utf16(char *haystack, size_t haystack_len, char *needle,
 	if(char_pos != USEARCH_DONE && ubrk_isBoundary(bi, char_pos)) {
 		ret_pos = grapheme_count_graphemes(bi, uhaystack,char_pos);
 		if(puchar_pos) {
-			*puchar_pos = char_pos;
+			*puchar_pos = u_countChar32(uhaystack, char_pos);
 		}
 	} else {
 		ret_pos = -1;
diff --git a/ext/intl/tests/grapheme_strstr_supplementary_prefix.phpt b/ext/intl/tests/grapheme_strstr_supplementary_prefix.phpt
new file mode 100644
index 000000000000..f9f85880604a
--- /dev/null
+++ b/ext/intl/tests/grapheme_strstr_supplementary_prefix.phpt
@@ -0,0 +1,27 @@
+--TEST--
+grapheme_strstr() and grapheme_stristr() preserve match boundaries after supplementary characters
+--EXTENSIONS--
+intl
+--FILE--
+<?php
+foreach (["\u{1F600}", "\u{1F600}\u{1F600}", "\u{1F600}e\u{0301}"] as $prefix) {
+    $haystack = $prefix . 'abc';
+    var_dump(grapheme_strstr($haystack, 'a'));
+    var_dump(grapheme_strstr($haystack, 'a', true));
+    var_dump(grapheme_stristr($haystack, 'A'));
+    var_dump(grapheme_stristr($haystack, 'A', true));
+}
+?>
+--EXPECT--
+string(3) "abc"
+string(4) "😀"
+string(3) "abc"
+string(4) "😀"
+string(3) "abc"
+string(8) "😀😀"
+string(3) "abc"
+string(8) "😀😀"
+string(3) "abc"
+string(7) "😀é"
+string(3) "abc"
+string(7) "😀é"