diff --git a/src/wp-includes/html-api/class-wp-html-tag-processor.php b/src/wp-includes/html-api/class-wp-html-tag-processor.php
index 501a623afb10b..c7a545f8b2003 100644
--- a/src/wp-includes/html-api/class-wp-html-tag-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-tag-processor.php
@@ -627,6 +627,15 @@ class WP_HTML_Tag_Processor {
 	 */
 	private $token_length;
 
+	/**
+	 * Whether the current tag token has the self-closing flag.
+	 *
+	 * @since 7.1.0
+	 *
+	 * @var bool
+	 */
+	private $self_closing_flag = false;
+
 	/**
 	 * Byte offset in input document where current tag name starts.
 	 *
@@ -1079,6 +1088,7 @@ private function base_class_next_token(): bool {
 		$tag_name_starts_at   = $this->tag_name_starts_at;
 		$tag_name_length      = $this->tag_name_length;
 		$tag_ends_at          = $this->token_starts_at + $this->token_length;
+		$self_closing_flag    = $this->self_closing_flag;
 		$attributes           = $this->attributes;
 		$duplicate_attributes = $this->duplicate_attributes;
 
@@ -1136,6 +1146,7 @@ private function base_class_next_token(): bool {
 		$this->text_length          = $this->tag_name_starts_at - $this->text_starts_at;
 		$this->tag_name_starts_at   = $tag_name_starts_at;
 		$this->tag_name_length      = $tag_name_length;
+		$this->self_closing_flag    = $self_closing_flag;
 		$this->attributes           = $attributes;
 		$this->duplicate_attributes = $duplicate_attributes;
 
@@ -2143,7 +2154,19 @@ private function parse_next_attribute(): bool {
 		$doc_length = strlen( $this->html );
 
 		// Skip whitespace and slashes.
-		$this->bytes_already_parsed += strspn( $this->html, " \t\f\r\n/", $this->bytes_already_parsed );
+		$skipped_start               = $this->bytes_already_parsed;
+		$this->bytes_already_parsed += strspn( $this->html, " \t\f\r\n/", $skipped_start );
+
+		// A slash inside an unquoted attribute value will not have been skipped here.
+		if (
+			$this->bytes_already_parsed < $doc_length &&
+			$this->bytes_already_parsed > $skipped_start &&
+			'/' === $this->html[ $this->bytes_already_parsed - 1 ] &&
+			'>' === $this->html[ $this->bytes_already_parsed ]
+		) {
+			$this->self_closing_flag = true;
+		}
+
 		if ( $this->bytes_already_parsed >= $doc_length ) {
 			$this->parser_state = self::STATE_INCOMPLETE_INPUT;
 
@@ -2327,6 +2350,7 @@ private function after_tag(): void {
 
 		$this->token_starts_at          = null;
 		$this->token_length             = null;
+		$this->self_closing_flag        = false;
 		$this->tag_name_starts_at       = null;
 		$this->tag_name_length          = null;
 		$this->text_starts_at           = 0;
@@ -3335,15 +3359,7 @@ public function has_self_closing_flag(): bool {
 			return false;
 		}
 
-		/*
-		 * The self-closing flag is the solidus at the _end_ of the tag, not the beginning.
-		 *
-		 * Example:
-		 *
-		 *     <figure />
-		 *             ^ this appears one character before the end of the closing ">".
-		 */
-		return '/' === $this->html[ $this->token_starts_at + $this->token_length - 2 ];
+		return $this->self_closing_flag;
 	}
 
 	/**
diff --git a/tests/phpunit/tests/html-api/wpHtmlProcessor.php b/tests/phpunit/tests/html-api/wpHtmlProcessor.php
index a89014282df73..30de6d406468e 100644
--- a/tests/phpunit/tests/html-api/wpHtmlProcessor.php
+++ b/tests/phpunit/tests/html-api/wpHtmlProcessor.php
@@ -583,6 +583,29 @@ public function test_expects_closer_foreign_content_self_closing() {
 		$this->assertTrue( $processor->expects_closer() );
 	}
 
+	/**
+	 * Ensures a trailing slash in an unquoted attribute value does not close foreign content.
+	 *
+	 * @ticket 61576
+	 */
+	public function test_trailing_slash_in_unquoted_attribute_value_does_not_self_close_foreign_content() {
+		$processor = WP_HTML_Processor::create_fragment( '<math><mtext a=b/>This mtext tag is not self-closing, it has [a="b/"] attribute.' );
+
+		$this->assertTrue( $processor->next_tag( 'MTEXT' ), 'Could not find MTEXT tag: check test setup.' );
+		$this->assertSame( 'b/', $processor->get_attribute( 'a' ), 'Trailing slash in unquoted attribute value should belong to the attribute value.' );
+		$this->assertFalse(
+			$processor->has_self_closing_flag(),
+			'Trailing slash in unquoted attribute value should not be interpreted as a self-closing flag.'
+		);
+
+		$this->assertTrue( $processor->next_token(), 'Could not find text following MTEXT tag: check test setup.' );
+		$this->assertSame(
+			array( 'HTML', 'BODY', 'MATH', 'MTEXT', '#text' ),
+			$processor->get_breadcrumbs(),
+			'Text following the MTEXT tag should remain inside the MTEXT element.'
+		);
+	}
+
 	/**
 	 * Ensures that expects_closer works for void-like elements in foreign content.
 	 *
diff --git a/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php b/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
index 22ace3890f469..949cfa1b2a62d 100644
--- a/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
+++ b/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
@@ -111,10 +111,12 @@ public static function data_has_self_closing_flag() {
 			'No self-closing flag on a foreign element'  => array( '<circle>', false ),
 			// These involve syntax peculiarities.
 			'Self-closing flag after extra spaces'       => array( '<div      />', true ),
-			'Self-closing flag after attribute'          => array( '<div id=test/>', true ),
+			'Self-closing flag after attribute'          => array( '<div id=test />', true ),
+			'Slash inside unquoted attribute value'      => array( '<div id=test/>', false ),
 			'Self-closing flag after quoted attribute'   => array( '<div id="test"/>', true ),
 			'Self-closing flag after boolean attribute'  => array( '<div enabled/>', true ),
 			'Boolean attribute that looks like a self-closer' => array( '<div / >', false ),
+			'Self-closing flag on internally consumed special element closer' => array( '<title>x</title/>', false ),
 		);
 	}