diff --git a/packages/block-library/src/post-comments/block.json b/packages/block-library/src/post-comments/block.json
index 04915d0b4c0ac..51cbd0018560a 100644
--- a/packages/block-library/src/post-comments/block.json
+++ b/packages/block-library/src/post-comments/block.json
@@ -31,5 +31,9 @@
 			"link": true
 		}
 	},
-	"style": "wp-block-post-comments"
+	"style": [
+		"wp-block-post-comments",
+		"wp-block-buttons",
+		"wp-block-button"
+	]
 }
diff --git a/packages/block-library/src/post-comments/index.php b/packages/block-library/src/post-comments/index.php
index 42f9a8d60d94e..913ff8347fff9 100644
--- a/packages/block-library/src/post-comments/index.php
+++ b/packages/block-library/src/post-comments/index.php
@@ -66,3 +66,18 @@ function register_block_core_post_comments() {
 	);
 }
 add_action( 'init', 'register_block_core_post_comments' );
+
+/**
+ * Use the button block classes for the form-submit button.
+ *
+ * @param array $fields The default comment form arguments.
+ *
+ * @return array Returns the modified fields.
+ */
+function gutenberg_post_comments_block_form_defaults( $fields ) {
+	$fields['submit_button'] = '<input name="%1$s" type="submit" id="%2$s" class="%3$s wp-block-button__link" value="%4$s" />';
+	$fields['submit_field']  = '<p class="form-submit wp-block-button">%1$s %2$s</p>';
+
+	return $fields;
+}
+add_filter( 'comment_form_defaults', 'gutenberg_post_comments_block_form_defaults' );