diff --git a/lib/experimental/blocks.php b/lib/experimental/blocks.php
index a33d2f6b98012..dad4ef971d43f 100644
--- a/lib/experimental/blocks.php
+++ b/lib/experimental/blocks.php
@@ -100,3 +100,108 @@ function gutenberg_register_metadata_attribute( $args ) {
 	return $args;
 }
 add_filter( 'register_block_type_args', 'gutenberg_register_metadata_attribute' );
+
+/**
+ * Return a function that auto-inserts as the first or last inner block of a given block.
+ *
+ * @param string $relative_position The position relative to the given block ("first_child" or "last_child").
+ * @param array  $inserted_block    The block to insert.
+ * @return callable A function that accepts a block's content and returns the content with the inserted block.
+ */
+function gutenberg_auto_insert_child_block( $relative_position, $inserted_block ) {
+	return function( $parsed_block ) use ( $relative_position, $inserted_block ) {
+		if ( 'first_child' === $relative_position ) {
+			array_unshift( $parsed_block['innerBlocks'], $inserted_block );
+			// Since WP_Block::render() iterates over `inner_content` (rather than `inner_blocks`)
+			// when rendering blocks, we also need to prepend a value (`null`, to mark a block
+			// location) to that array.
+			array_unshift( $parsed_block['innerContent'], null );
+		} elseif ( 'last_child' === $relative_position ) {
+			array_push( $parsed_block['innerBlocks'], $inserted_block );
+			// Since WP_Block::render() iterates over `inner_content` (rather than `inner_blocks`)
+			// when rendering blocks, we also need to prepend a value (`null`, to mark a block
+			// location) to that array.
+			array_push( $parsed_block['innerContent'], null );
+		}
+		return $parsed_block;
+	};
+}
+
+/**
+ * Return a function that auto-inserts blocks relative to a given block.
+ *
+ * @param string $relative_position The position relative to the given block.
+ * @param array  $inserted_block    The block to insert.
+ * @return callable A function that accepts a block's content and returns the content with the inserted block.
+ */
+function gutenberg_auto_insert_block( $relative_position, $inserted_block ) {
+	// Can we avoid infinite loops?
+
+	return function( $block_content ) use ( $relative_position, $inserted_block ) {
+		$inserted_content = render_block( $inserted_block );
+
+		if ( 'before' === $relative_position ) {
+			$block_content = $inserted_content . $block_content;
+		} elseif ( 'after' === $relative_position ) {
+			$block_content = $block_content . $inserted_content;
+		}
+		return $block_content;
+	};
+}
+
+function gutenberg_register_auto_inserted_blocks( $settings, $metadata ) {
+	if ( ! isset( $metadata['autoInsert'] ) ) {
+		return $settings;
+	}
+
+	$property_mappings = array(
+		'before'     => 'before',
+		'after'      => 'after',
+		'firstChild' => 'first_child',
+		'lastChild'  => 'last_child',
+	);
+
+	$auto_insert = $metadata['autoInsert'];
+	foreach ( $auto_insert as $block_name => $block_data ) {
+		$position = $block_data['position'];
+		if ( ! isset( $property_mappings[ $position ] ) ) {
+			continue;
+		}
+
+		$mapped_position = $property_mappings[ $position ];
+
+		$inserted_block = array(
+			'blockName' => $metadata['name'],
+			'attrs'     => $block_data['attrs'],
+		);
+		// TODO: In the long run, we'd likely want some sort of registry for auto-inserted blocks.
+		if ( 'before' === $mapped_position || 'after' === $mapped_position ) {
+			$inserter = gutenberg_auto_insert_block( $mapped_position, $inserted_block );
+			add_filter( "render_block_$block_name", $inserter, 10, 2 );
+		} elseif ( 'first_child' === $mapped_position || 'last_child' === $mapped_position ) {
+			$inserter = gutenberg_auto_insert_child_block( $mapped_position, $inserted_block );
+			add_filter( "render_block_data_$block_name", $inserter, 10, 2 );
+		}
+		$settings['auto_insert'][ $block_name ] = $mapped_position;
+	}
+
+	return $settings;
+}
+add_filter( 'block_type_metadata_settings', 'gutenberg_register_auto_inserted_blocks', 10, 2 );
+
+function gutenberg_apply_render_block_data_block_type_filter( $parsed_block, $source_block, $parent_block ) {
+	$block_name = $parsed_block['blockName'];
+	/**
+	 * Filters the block being rendered in render_block(), before it's processed.
+	 *
+	 * The dynamic portion of the hook name, `$name`, refers to
+	 * the block name, e.g. "core/paragraph".
+	 *
+	 * @param array         $parsed_block The block being rendered.
+	 * @param array         $source_block An un-modified copy of $parsed_block, as it appeared in the source content.
+	 * @param WP_Block|null $parent_block If this is a nested block, a reference to the parent block.
+	 */
+	$parsed_block = apply_filters( "render_block_data_$block_name", $parsed_block, $source_block, $parent_block );
+	return $parsed_block;
+}
+add_filter( 'render_block_data', 'gutenberg_apply_render_block_data_block_type_filter', 15, 3 );
diff --git a/packages/block-library/src/avatar/block.json b/packages/block-library/src/avatar/block.json
index 3fbb6dd9221ae..902c3300bfec5 100644
--- a/packages/block-library/src/avatar/block.json
+++ b/packages/block-library/src/avatar/block.json
@@ -50,5 +50,18 @@
 		}
 	},
 	"editorStyle": "wp-block-avatar",
-	"style": "wp-block-avatar"
+	"style": "wp-block-avatar",
+	"autoInsert": {
+		"core/comment-template": {
+			"position": "lastChild",
+			"attrs": {
+				"size": 40,
+				"style": {
+					"border": {
+						"radius": "10px"
+					}
+				}
+			}
+		}
+	}
 }
diff --git a/packages/block-library/src/social-link/block.json b/packages/block-library/src/social-link/block.json
index 140cc123ec484..3ae1fe41819ba 100644
--- a/packages/block-library/src/social-link/block.json
+++ b/packages/block-library/src/social-link/block.json
@@ -34,5 +34,14 @@
 		"reusable": false,
 		"html": false
 	},
-	"editorStyle": "wp-block-social-link-editor"
+	"editorStyle": "wp-block-social-link-editor",
+	"autoInsert": {
+		"core/post-content": {
+			"position": "after",
+			"attrs": {
+				"service": "wordpress",
+				"url": "https://wordpress.org/"
+			}
+		}
+	}
 }
diff --git a/phpunit/blocks/render-comment-template-test.php b/phpunit/blocks/render-comment-template-test.php
index c297d1729d0dc..19f453fc18594 100644
--- a/phpunit/blocks/render-comment-template-test.php
+++ b/phpunit/blocks/render-comment-template-test.php
@@ -142,6 +142,9 @@ public function test_inner_block_inserted_by_render_block_data_is_retained() {
 			return $parsed_block;
 		};
 
+		// Remove auto-insertion filter so it won't collide.
+		remove_filter( 'render_block_data', 'gutenberg_auto_insert_child_block' );
+
 		add_filter( 'render_block_data', $render_block_data_callback, 10, 1 );
 		$parsed_blocks = parse_blocks(
 			'<!-- wp:comments --><!-- wp:comment-template --><!-- wp:comment-content /--><!-- /wp:comment-template --><!-- /wp:comments -->'
@@ -154,6 +157,8 @@ public function test_inner_block_inserted_by_render_block_data_is_retained() {
 		);
 		$block->render();
 		remove_filter( 'render_block_data', $render_block_data_callback );
+		// Add back auto-insertion filter.
+		add_filter( 'render_block_data', 'gutenberg_auto_insert_child_block', 10, 1 );
 
 		$this->assertSame( 5, $render_block_callback->get_call_count() );
 
diff --git a/schemas/json/block.json b/schemas/json/block.json
index 5b92a654fbc4a..14a560b4a0070 100644
--- a/schemas/json/block.json
+++ b/schemas/json/block.json
@@ -737,6 +737,32 @@
 		"render": {
 			"type": "string",
 			"description": "Template file loaded on the server when rendering a block."
+		},
+		"autoInsert": {
+			"type": "object",
+			"description": "Blocks to auto-insert this block next to.",
+			"patternProperties": {
+				"[a-zA-Z]": {
+					"type": "object",
+					"description": "Position relative to the block to auto-insert this block next to.",
+					"properties": {
+						"position": {
+							"type": "string",
+							"description": "Position relative to the block to auto-insert this block next to.",
+							"enum": [
+								"before",
+								"after",
+								"firstChild",
+								"lastChild"
+							]
+						},
+						"attrs": {
+							"type": "object",
+							"description": "Attributes for the auto-inserted block."
+						}
+					}
+				}
+			}
 		}
 	},
 	"required": [ "name", "title" ],