diff --git a/packages/blocks/src/api/parser.js b/packages/blocks/src/api/parser.js
index b40532957c5f0..cb3a35d5fffe5 100644
--- a/packages/blocks/src/api/parser.js
+++ b/packages/blocks/src/api/parser.js
@@ -383,6 +383,7 @@ export function createBlockWithFallback( blockNode ) {
 		innerBlocks = [],
 		innerHTML,
 	} = blockNode;
+	const { innerContent } = blockNode;
 	const freeformContentFallbackBlock = getFreeformContentHandlerName();
 	const unregisteredFallbackBlock = getUnregisteredTypeHandlerName() || freeformContentFallbackBlock;
 
@@ -416,17 +417,39 @@ export function createBlockWithFallback( blockNode ) {
 	let blockType = getBlockType( name );
 
 	if ( ! blockType ) {
-		// Preserve undelimited content for use by the unregistered type handler.
-		const originalUndelimitedContent = innerHTML;
+		// Since the constituents of the block node are extracted at the start
+		// of the present function, construct a new object rather than reuse
+		// `blockNode`.
+		const reconstitutedBlockNode = {
+			attrs: attributes,
+			blockName: originalName,
+			innerBlocks,
+			innerContent,
+		};
+
+		// Preserve undelimited content for use by the unregistered type
+		// handler. A block node's `innerHTML` isn't enough, as that field only
+		// carries the block's own HTML and not its nested blocks'.
+		const originalUndelimitedContent = serializeBlockNode(
+			reconstitutedBlockNode,
+			{ isCommentDelimited: false }
+		);
+
+		// Preserve full block content for use by the unregistered type
+		// handler, block boundaries included.
+		const originalContent = serializeBlockNode(
+			reconstitutedBlockNode,
+			{ isCommentDelimited: true }
+		);
 
 		// If detected as a block which is not registered, preserve comment
 		// delimiters in content of unregistered type handler.
 		if ( name ) {
-			innerHTML = getCommentDelimitedContent( name, attributes, innerHTML );
+			innerHTML = originalContent;
 		}
 
 		name = unregisteredFallbackBlock;
-		attributes = { originalName, originalUndelimitedContent };
+		attributes = { originalName, originalContent, originalUndelimitedContent };
 		blockType = getBlockType( name );
 	}
 
@@ -457,15 +480,53 @@ export function createBlockWithFallback( blockNode ) {
 		block.isValid = isValidBlockContent( blockType, block.attributes, innerHTML );
 	}
 
-	// Preserve original content for future use in case the block is parsed as
-	// invalid, or future serialization attempt results in an error.
-	block.originalContent = innerHTML;
+	// Preserve original content for future use in case the block is parsed
+	// as invalid, or future serialization attempt results in an error.
+	block.originalContent = block.originalContent || innerHTML;
 
 	block = getMigratedBlock( block, attributes );
 
 	return block;
 }
 
+/**
+ * Serializes a block node into the native HTML-comment-powered block format.
+ * CAVEAT: This function is intended for reserializing blocks as parsed by
+ * valid parsers and skips any validation steps. This is NOT a generic
+ * serialization function for in-memory blocks. For most purposes, see the
+ * following functions available in the `@wordpress/blocks` package:
+ *
+ * @see serializeBlock
+ * @see serialize
+ *
+ * For more on the format of block nodes as returned by valid parsers:
+ *
+ * @see `@wordpress/block-serialization-default-parser` package
+ * @see `@wordpress/block-serialization-spec-parser` package
+ *
+ * @param {Object}   blockNode                  A block node as returned by a valid parser.
+ * @param {?Object}  options                    Serialization options.
+ * @param {?boolean} options.isCommentDelimited Whether to output HTML comments around blocks.
+ *
+ * @return {string} An HTML string representing a block.
+ */
+export function serializeBlockNode( blockNode, options = {} ) {
+	const { isCommentDelimited = true } = options;
+	const { blockName, attrs = {}, innerBlocks = [], innerContent = [] } = blockNode;
+
+	let childIndex = 0;
+	const content = innerContent.map( ( item ) =>
+		// `null` denotes a nested block, otherwise we have an HTML fragment
+		item !== null ?
+			item :
+			serializeBlockNode( innerBlocks[ childIndex++ ], options )
+	).join( '\n' ).replace( /\n+/g, '\n' ).trim();
+
+	return isCommentDelimited ?
+		getCommentDelimitedContent( blockName, attrs, content ) :
+		content;
+}
+
 /**
  * Creates a parse implementation for the post content which returns a list of blocks.
  *
diff --git a/packages/blocks/src/api/test/parser.js b/packages/blocks/src/api/test/parser.js
index 8c0e50feee3ee..f10001cb0c013 100644
--- a/packages/blocks/src/api/test/parser.js
+++ b/packages/blocks/src/api/test/parser.js
@@ -19,6 +19,7 @@ import {
 	isOfTypes,
 	isValidByType,
 	isValidByEnum,
+	serializeBlockNode,
 } from '../parser';
 import {
 	registerBlockType,
@@ -757,6 +758,111 @@ describe( 'block parser', () => {
 		} );
 	} );
 
+	describe( 'serializeBlockNode', () => {
+		it( 'reserializes block nodes', () => {
+			const expected = `<!-- wp:columns -->
+				<div class="wp-block-columns has-2-columns">
+					<!-- wp:column -->
+					<div class="wp-block-column">
+						<!-- wp:paragraph -->
+						<p>A</p>
+						<!-- /wp:paragraph -->
+					</div>
+					<!-- /wp:column -->
+					<!-- wp:column -->
+					<div class="wp-block-column">
+						<!-- wp:group -->
+						<div class="wp-block-group"><div class="wp-block-group__inner-container">
+							<!-- wp:list -->
+							<ul><li>B</li><li>C</li></ul>
+							<!-- /wp:list -->
+							<!-- wp:paragraph -->
+							<p>D</p>
+							<!-- /wp:paragraph -->
+						</div></div>
+						<!-- /wp:group -->
+					</div>
+					<!-- /wp:column -->
+				</div>
+				<!-- /wp:columns -->`.replace( /\t/g, '' );
+			const input = {
+				blockName: 'core/columns',
+				attrs: {},
+				innerBlocks: [
+					{
+						blockName: 'core/column',
+						attrs: {},
+						innerBlocks: [
+							{
+								blockName: 'core/paragraph',
+								attrs: {},
+								innerBlocks: [],
+								innerHTML: '<p>A</p>',
+								innerContent: [ '<p>A</p>' ],
+							},
+						],
+						innerHTML: '<div class="wp-block-column"></div>',
+						innerContent: [
+							'<div class="wp-block-column">',
+							null,
+							'</div>',
+						],
+					},
+					{
+						blockName: 'core/column',
+						attrs: {},
+						innerBlocks: [
+							{
+								blockName: 'core/group',
+								attrs: {},
+								innerBlocks: [
+									{
+										blockName: 'core/list',
+										attrs: {},
+										innerBlocks: [],
+										innerHTML: '<ul><li>B</li><li>C</li></ul>',
+										innerContent: [ '<ul><li>B</li><li>C</li></ul>' ],
+									},
+									{
+										blockName: 'core/paragraph',
+										attrs: {},
+										innerBlocks: [],
+										innerHTML: '<p>D</p>',
+										innerContent: [ '<p>D</p>' ],
+									},
+								],
+								innerHTML: '<div class="wp-block-group"><div class="wp-block-group__inner-container"></div></div>',
+								innerContent: [
+									'<div class="wp-block-group"><div class="wp-block-group__inner-container">',
+									null,
+									'',
+									null,
+									'</div></div>' ],
+							},
+						],
+						innerHTML: '<div class="wp-block-column"></div>',
+						innerContent: [
+							'<div class="wp-block-column">',
+							null,
+							'</div>',
+						],
+					},
+				],
+				innerHTML: '<div class="wp-block-columns has-2-columns"></div>',
+				innerContent: [
+					'<div class="wp-block-columns has-2-columns">',
+					null,
+					'',
+					null,
+					'</div>',
+				],
+			};
+			const actual = serializeBlockNode( input );
+
+			expect( actual ).toEqual( expected );
+		} );
+	} );
+
 	describe( 'parse() of @wordpress/block-serialization-spec-parser', () => {
 		// run the test cases using the PegJS defined parser
 		testCases( parsePegjs );