diff --git a/NEWS b/NEWS
index 612cd9e9a39..a09a5dfda79 100644
--- a/NEWS
+++ b/NEWS
@@ -17,6 +17,10 @@ PHP                                                                        NEWS
   . Fixed bug GH-20668 (\Uri\WhatWg\Url::withHost() crashes (SEGV) for URLs
     using the file: scheme). (lexborisov)
 
+- Mbstring
+  . Fixed bug GH-20674 (Fix GH-20674 mb_decode_mimeheader does not handle
+    separator). (Yuya Hamada)
+
 - Sqlite3:
   . Fixed bug GH-20699 (SQLite3Result fetchArray return array|false,
     null returned). (ndossche, plusminmax)
diff --git a/ext/mbstring/mbstring.c b/ext/mbstring/mbstring.c
index 4044f4de9ff..9f946508464 100644
--- a/ext/mbstring/mbstring.c
+++ b/ext/mbstring/mbstring.c
@@ -6667,13 +6667,15 @@ static zend_string* mb_mime_header_decode(zend_string *input, const mbfl_encodin
 					p = temp;
 					/* Decoding of MIME encoded word was successful;
 					 * Try to collapse a run of whitespace */
-					if (p < e && (*p == '\n' || *p == '\r')) {
+					if (p < e && (*p == '\n' || *p == '\r' || *p == '\t' || *p == ' ')) {
 						do {
 							p++;
 						} while (p < e && (*p == '\n' || *p == '\r' || *p == '\t' || *p == ' '));
 						/* We will only actually output a space if this is not immediately followed
 						 * by another valid encoded word */
 						space_pending = true;
+					} else {
+						space_pending = false;
 					}
 					continue;
 				}
diff --git a/ext/mbstring/tests/gh20674.phpt b/ext/mbstring/tests/gh20674.phpt
new file mode 100644
index 00000000000..2fb8206037d
--- /dev/null
+++ b/ext/mbstring/tests/gh20674.phpt
@@ -0,0 +1,40 @@
+--TEST--
+GH-20674 (mb_decode_mimeheader does not handle separator)
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+
+$subject = '=?us-ascii?Q?The_PH?=  =?us-ascii?Q?P_8.5?=';
+var_dump(mb_decode_mimeheader($subject));
+
+// mb_decode_mimeheader's backward compatible for TAB(\t)
+$subject = "=?us-ascii?Q?The_PH?=\t=?us-ascii?Q?P_8.5?=";
+var_dump(mb_decode_mimeheader($subject));
+
+$subject = "=?us-ascii?Q?The_PH?=\t =?us-ascii?Q?P_8.5?=";
+var_dump(mb_decode_mimeheader($subject));
+
+$subject = "=?us-ascii?Q?The_PH?= \t =?us-ascii?Q?P_8.5?=";
+var_dump(mb_decode_mimeheader($subject));
+
+// from RFC 2047 https://www.ietf.org/rfc/rfc2047#section-8
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?=)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?= b)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a_b?=)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?= =?ISO-8859-1?Q?b?=)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?=  =?ISO-8859-1?Q?b?=)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?=
+	=?ISO-8859-1?Q?b?=)"));
+?>
+--EXPECTF--
+string(11) "The PHP 8.5"
+string(11) "The PHP 8.5"
+string(11) "The PHP 8.5"
+string(11) "The PHP 8.5"
+string(3) "(a)"
+string(5) "(a b)"
+string(5) "(a b)"
+string(4) "(ab)"
+string(4) "(ab)"
+string(4) "(ab)"