diff --git a/packages/blocks/src/api/serializer.js b/packages/blocks/src/api/serializer.js
index c249df7251db84..8c073c4d6d1791 100644
--- a/packages/blocks/src/api/serializer.js
+++ b/packages/blocks/src/api/serializer.js
@@ -24,8 +24,11 @@ import {
 	getFreeformContentHandlerName,
 	getUnregisteredTypeHandlerName,
 } from './registration';
+import { serializeRawBlock } from './parser/serialize-raw-block';
 import { isUnmodifiedDefaultBlock, normalizeBlockType } from './utils';
 
+/** @typedef {import('./parser').WPBlock} WPBlock */
+
 /**
  * @typedef {Object} WPBlockSerializationOptions Serialization Options.
  *
@@ -337,12 +340,16 @@ export function getCommentDelimitedContent(
  * Returns the content of a block, including comment delimiters, determining
  * serialized attributes and content form from the current state of the block.
  *
- * @param {Object}                      block   Block instance.
- * @param {WPBlockSerializationOptions} options Serialization options.
+ * @param {WPBlock}                      block   Block instance.
+ * @param {WPBlockSerializationOptions}  options Serialization options.
  *
  * @return {string} Serialized block.
  */
 export function serializeBlock( block, { isInnerBlocks = false } = {} ) {
+	if ( ! block.isValid && block.__unstableBlockSource ) {
+		return serializeRawBlock( block.__unstableBlockSource );
+	}
+
 	const blockName = block.name;
 	const saveContent = getBlockInnerHTML( block );
 
diff --git a/packages/blocks/src/api/test/serializer.js b/packages/blocks/src/api/test/serializer.js
index 513eadeb84104c..22fa5bc7b6dc6a 100644
--- a/packages/blocks/src/api/test/serializer.js
+++ b/packages/blocks/src/api/test/serializer.js
@@ -307,6 +307,48 @@ describe( 'block serializer', () => {
 
 			expect( content ).toBe( 'Bananas' );
 		} );
+		it( 'preserves content from invalid blocks when source information is present', () => {
+			registerBlockType( 'core/quote', {
+				category: 'text',
+				title: 'Quote',
+				attributes: { content: 'string' },
+				save: ( { attributes } ) =>
+					createElement( 'blockquote', {}, attributes.content ),
+			} );
+
+			const block = {
+				...createBlock( 'core/quote' ),
+				isValid: false,
+				__unstableBlockSource: {
+					blockName: 'quote',
+					attrs: {},
+					innerHTML: '<p>Not a quote</p>',
+					innerBlocks: [],
+					innerContent: [ '<p>Not a quote</p>' ],
+				},
+			};
+
+			expect( serializeBlock( block ) ).toBe(
+				'<!-- wp:quote -->\n<p>Not a quote</p>\n<!-- /wp:quote -->'
+			);
+		} );
+		it( 're-generates content from invalid blocks when source information is missing (losing content)', () => {
+			registerBlockType( 'core/quote', {
+				category: 'text',
+				title: 'Quote',
+				attributes: { content: 'string' },
+				save: ( { attributes } ) =>
+					createElement( 'blockquote', {}, attributes.content ),
+			} );
+
+			// missing attributes as a result of a failed parse
+			const block = {
+				...createBlock( 'core/quote' ),
+				isValid: false,
+			};
+
+			expect( serializeBlock( block ) ).toBe( '<!-- wp:quote /-->' );
+		} );
 	} );
 
 	describe( 'serialize()', () => {