diff --git a/NEWS b/NEWS
index 75d2b9562e5..b3039bb92c1 100644
--- a/NEWS
+++ b/NEWS
@@ -47,6 +47,10 @@ PHP                                                                        NEWS
   . mb_detect_encoding's "non-strict" mode now behaves as described in the
     documentation. Previously, it would return false if the very first byte
     of the input string was invalid in all candidate encodings. (Alex Dowad)
+  . mb_strtolower, mb_strtotitle, and mb_convert_case implement conditional
+    casing rules for the Greek letter sigma. For mb_convert_case, conditional
+    casing only applies to MB_CASE_LOWER and MB_CASE_TITLE modes, not to
+    MB_CASE_LOWER_SIMPLE and MB_CASE_TITLE_SIMPLE. (Alex Dowad)
 
 - Opcache:
   . Added start, restart and force restart time to opcache's
diff --git a/UPGRADING b/UPGRADING
index f3254ab6781..e5e4e7c210d 100644
--- a/UPGRADING
+++ b/UPGRADING
@@ -56,6 +56,12 @@ PHP 8.3 UPGRADE NOTES
     "buffer_size" => int
     See GH-9336
 
+- MBString:
+  . mb_strtolower, mb_strtotitle, and mb_convert_case implement conditional
+    casing rules for the Greek letter sigma. For mb_convert_case, conditional
+    casing only applies to MB_CASE_LOWER and MB_CASE_TITLE modes, not to
+    MB_CASE_LOWER_SIMPLE and MB_CASE_TITLE_SIMPLE. (Alex Dowad)
+
 - Standard:
   . E_NOTICEs emitted by unserialized() have been promoted to E_WARNING.
     RFC: https://wiki.php.net/rfc/improve_unserialize_error_handling
diff --git a/ext/mbstring/php_unicode.c b/ext/mbstring/php_unicode.c
index b1ffb06c31f..3e4b683924b 100644
--- a/ext/mbstring/php_unicode.c
+++ b/ext/mbstring/php_unicode.c
@@ -238,6 +238,45 @@ static uint32_t *emit_special_casing_sequence(uint32_t w, uint32_t *out)
 	return out;
 }
 
+/* Used when determining whether special casing rules should be applied to Greek letter sigma */
+static bool scan_ahead_for_cased_letter(unsigned char *in, size_t in_len, unsigned int state, const mbfl_encoding *encoding)
+{
+	uint32_t wchar_buf[64];
+
+	while (in_len) {
+		size_t out_len = encoding->to_wchar(&in, &in_len, wchar_buf, 64, &state);
+		ZEND_ASSERT(out_len <= 64);
+		for (unsigned int i = 0; i < out_len; i++) {
+			uint32_t w = wchar_buf[i];
+			if (php_unicode_is_cased(w)) {
+				return true;
+			}
+			if (!php_unicode_is_case_ignorable(w)) {
+				return false;
+			}
+		}
+	}
+
+	return false;
+}
+
+/* Used when determining whether special casing rules should be applied to Greek letter sigma */
+static bool scan_back_for_cased_letter(uint32_t *begin, uint32_t *end)
+{
+	if (end != NULL) {
+		while (--end >= begin) {
+			uint32_t w = *end;
+			if (php_unicode_is_cased(w)) {
+				return true;
+			}
+			if (!php_unicode_is_case_ignorable(w)) {
+				return false;
+			}
+		}
+	}
+	return false;
+}
+
 MBSTRING_API zend_string *php_unicode_convert_case(php_case_mode case_mode, const char *srcstr, size_t in_len, const mbfl_encoding *src_encoding, const mbfl_encoding *dst_encoding, int illegal_mode, uint32_t illegal_substchar)
 {
 	/* A Unicode codepoint can expand out to up to 3 codepoints when uppercased, lowercased, or title cased
@@ -246,6 +285,9 @@ MBSTRING_API zend_string *php_unicode_convert_case(php_case_mode case_mode, cons
 	unsigned int state = 0, title_mode = 0;
 	unsigned char *in = (unsigned char*)srcstr;
 	enum mbfl_no_encoding enc = src_encoding->no_encoding;
+	/* In rare cases, we need to scan backwards through the previously converted codepoints to see
+	 * if special conversion rules should be used for the Greek letter sigma */
+	uint32_t *converted_end = NULL;
 
 	mb_convert_buf buf;
 	mb_convert_buf_init(&buf, in_len + 1, illegal_substchar, illegal_mode);
@@ -315,6 +357,43 @@ MBSTRING_API zend_string *php_unicode_convert_case(php_case_mode case_mode, cons
 					*p++ = w;
 					continue;
 				}
+				if (w == 0x3A3) {
+					/* For Greek capital letter sigma, there is a special casing rule;
+					 * if it is the last letter in a word, it should be downcased to U+03C2
+					 * (GREEK SMALL LETTER FINAL SIGMA)
+					 * Specifically, we need to check if this codepoint is preceded by any
+					 * number of case-ignorable codepoints, preceded by a cased letter, AND
+					 * is NOT followed by any number of case-ignorable codepoints followed
+					 * by a cased letter.
+					 * Ref: http://www.unicode.org/reports/tr21/tr21-5.html
+					 * Ref: https://unicode.org/Public/UNIDATA/SpecialCasing.txt
+					 *
+					 * While the special casing rules say we should scan backwards through "any number"
+					 * of case-ignorable codepoints, that is a great implementation burden
+					 * It would basically mean we need to keep all the codepoints in a big buffer
+					 * during this conversion operation, but we don't want to do that (to reduce the
+					 * amount of temporary scratch memory used)
+					 * Hence, we only scan back through the codepoints in wchar_buf, and if we hit the
+					 * beginning of the buffer, whatever codepoints have not yet been overwritten in
+					 * the latter part of converted_buf */
+					int j = i - 1;
+					while (j >= 0 && php_unicode_is_case_ignorable(wchar_buf[j])) {
+						j--;
+					}
+					if (j >= 0 ? php_unicode_is_cased(wchar_buf[j]) : scan_back_for_cased_letter(p, converted_end)) {
+						/* Now scan ahead to look for a cased letter */
+						j = i + 1;
+						while (j < out_len && php_unicode_is_case_ignorable(wchar_buf[j])) {
+							j++;
+						}
+						/* If we hit the end of wchar_buf, convert more of the input string into
+						 * codepoints and continue scanning */
+						if (j >= out_len ? !scan_ahead_for_cased_letter(in, in_len, state, src_encoding) : !php_unicode_is_cased(wchar_buf[j])) {
+							*p++ = 0x3C2;
+							continue;
+						}
+					}
+				}
 				w = php_unicode_tolower_raw(w, enc);
 				if (UNEXPECTED(w > 0xFFFFFF)) {
 					p = emit_special_casing_sequence(w, p);
@@ -362,6 +441,7 @@ MBSTRING_API zend_string *php_unicode_convert_case(php_case_mode case_mode, cons
 			EMPTY_SWITCH_DEFAULT_CASE()
 		}
 
+		converted_end = p;
 		ZEND_ASSERT(p - converted_buf <= 192);
 		dst_encoding->from_wchar(converted_buf, p - converted_buf, &buf, !in_len);
 	}
diff --git a/ext/mbstring/tests/casemapping.phpt b/ext/mbstring/tests/casemapping.phpt
index 85f5140e2bf..85a21494a6f 100644
--- a/ext/mbstring/tests/casemapping.phpt
+++ b/ext/mbstring/tests/casemapping.phpt
@@ -58,6 +58,12 @@ echo bin2hex(mb_convert_case($str, MB_CASE_UPPER_SIMPLE)), "\n";
 echo bin2hex(mb_convert_case($str, MB_CASE_FOLD)), "\n";
 echo bin2hex(mb_convert_case($str, MB_CASE_FOLD_SIMPLE)), "\n";
 
+// Check handling of Greek letter capital sigma
+echo mb_convert_case("ΚΑΛΗΣΠΕΡΑ ΣΑΣ", MB_CASE_TITLE, "UTF-8"), "\n";
+echo mb_convert_case("ΚΑΛΗΣΠΕΡΑ ΣΑΣ", MB_CASE_TITLE_SIMPLE, "UTF-8"), "\n";
+echo mb_convert_case("ΚΑΛΗΣΠΕΡΑ ΣΑΣ", MB_CASE_LOWER, "UTF-8"), "\n";
+echo mb_convert_case("ΚΑΛΗΣΠΕΡΑ ΣΑΣ", MB_CASE_LOWER_SIMPLE, "UTF-8"), "\n";
+
 ?>
 --EXPECT--
 String: ß
@@ -109,3 +115,7 @@ dd
 dd
 69
 69
+Καλησπερα Σασ
+Καλησπερα Σασ
+καλησπερα σας
+καλησπερα σασ
diff --git a/ext/mbstring/tests/mb_strtolower_basic.phpt b/ext/mbstring/tests/mb_strtolower_basic.phpt
index debb42cd9ed..10b3282d33c 100644
--- a/ext/mbstring/tests/mb_strtolower_basic.phpt
+++ b/ext/mbstring/tests/mb_strtolower_basic.phpt
@@ -35,6 +35,33 @@ if ($mb == $greek_lower) {
     echo "Incorrectly converted\n";
 }
 
+echo "\n-- Greek letter sigma --\n";
+var_dump(mb_strtolower("Σ", 'UTF-8'));
+var_dump(mb_strtolower("aΣ", 'UTF-8'));
+var_dump(mb_strtolower("aΣb", 'UTF-8'));
+var_dump(mb_strtolower("aΣ b", 'UTF-8'));
+var_dump(mb_strtolower(" ΣΣΣΣ ", 'UTF-8'));
+
+// Apostrophe, full stop, colon, etc. are "case-ignorable"
+// When checking whether capital sigma is at the end of a word or not, we skip over
+// any number of case-ignorable characters, both when scanning back and when scanning forward
+var_dump(mb_strtolower("'Σ", 'UTF-8'));
+var_dump(mb_strtolower("ab'Σ", 'UTF-8'));
+var_dump(mb_strtolower("Σ'", 'UTF-8'));
+var_dump(mb_strtolower("Σ'a", 'UTF-8'));
+var_dump(mb_strtolower("a'Σ'a", 'UTF-8'));
+
+// We scan back by at least 63 characters when necessary,
+// but there is no guarantee that we will scan back further than that
+var_dump(mb_strtolower('a' . str_repeat('.', 63) . "Σ", 'UTF-8'));
+var_dump(mb_strtolower('a' . str_repeat('.', 64) . "Σ", 'UTF-8')); // Context-sensitive casing doesn't work here!
+
+// When scanning forward to confirm if capital sigma is at the end of a word or not,
+// there is no limit as to how far we will scan
+var_dump(mb_strtolower("abcΣ" . str_repeat('.', 64) . ' abc', 'UTF-8'));
+var_dump(mb_strtolower("abcΣ" . str_repeat('.', 64) . 'a abc', 'UTF-8'));
+var_dump(mb_strtolower("abcΣ" . str_repeat('.', 256) . ' abc', 'UTF-8'));
+
 echo "Done";
 ?>
 --EXPECT--
@@ -47,4 +74,21 @@ Correctly converted
 -- Multibyte String --
 string(64) "zrHOss6zzrTOtc62zrfOuM65zrrOu868zr3Ovs6/z4DPgc+Dz4TPhc+Gz4fPiM+J"
 Correctly converted
+
+-- Greek letter sigma --
+string(2) "σ"
+string(3) "aς"
+string(4) "aσb"
+string(5) "aς b"
+string(10) " σσσς "
+string(3) "'σ"
+string(5) "ab'ς"
+string(3) "σ'"
+string(4) "σ'a"
+string(6) "a'σ'a"
+string(66) "a...............................................................ς"
+string(67) "a................................................................σ"
+string(73) "abcς................................................................ abc"
+string(74) "abcσ................................................................a abc"
+string(265) "abcς................................................................................................................................................................................................................................................................ abc"
 Done