diff --git a/packages/editor/src/components/document-outline/index.js b/packages/editor/src/components/document-outline/index.js
index 6c498ccc79990..351d11f6d19c8 100644
--- a/packages/editor/src/components/document-outline/index.js
+++ b/packages/editor/src/components/document-outline/index.js
@@ -1,3 +1,8 @@
+/**
+ * External dependencies
+ */
+import clsx from 'clsx';
+
 /**
  * WordPress dependencies
  */
@@ -73,25 +78,55 @@ function EmptyOutlineIllustration() {
 	);
 }
 
+const incorrectMainTag = [
+	<br key="incorrect-break-main" />,
+	<em key="incorrect-message-main">
+		{ __(
+			'(Your template should have exactly one main element. Adjust the HTML element in the Advanced panel of the block.)'
+		) }
+	</em>,
+];
+
 /**
- * Returns an array of heading blocks enhanced with the following properties:
- * level   - An integer with the heading level.
- * isEmpty - Flag indicating if the heading has no content.
+ * Returns an array of heading blocks and blocks with the main tagName.
  *
- * @param {?Array} blocks An array of blocks.
+ * @param {?Array}  blocks            An array of blocks.
+ * @param {boolean} isShowingTemplate Indicates if a template is being edited or previewed.
  *
- * @return {Array} An array of heading blocks enhanced with the properties described above.
+ * @return {Array} An array of heading blocks and blocks with the main tagName.
  */
-const computeOutlineHeadings = ( blocks = [] ) => {
+const computeOutlineElements = ( blocks = [], isShowingTemplate = false ) => {
 	return blocks.flatMap( ( block = {} ) => {
-		if ( block.name === 'core/heading' ) {
+		const isHeading = block.name === 'core/heading';
+		const isMain =
+			isShowingTemplate && block.attributes?.tagName === 'main';
+
+		if ( isHeading ) {
 			return {
 				...block,
+				type: 'heading',
 				level: block.attributes.level,
 				isEmpty: isEmptyHeading( block ),
 			};
 		}
-		return computeOutlineHeadings( block.innerBlocks );
+
+		if ( isMain ) {
+			const children = computeOutlineElements(
+				block.innerBlocks || [],
+				isShowingTemplate
+			);
+			return [
+				{
+					...block,
+					type: 'main',
+					children,
+				},
+				// Flatten the children so that they are not nested under the main.
+				...children,
+			];
+		}
+
+		return computeOutlineElements( block.innerBlocks, isShowingTemplate );
 	} );
 };
 
@@ -113,104 +148,145 @@ export default function DocumentOutline( {
 	hasOutlineItemsDisabled,
 } ) {
 	const { selectBlock } = useDispatch( blockEditorStore );
-	const { blocks, title, isTitleSupported } = useSelect( ( select ) => {
-		const { getBlocks } = select( blockEditorStore );
-		const { getEditedPostAttribute } = select( editorStore );
-		const { getPostType } = select( coreStore );
-		const postType = getPostType( getEditedPostAttribute( 'type' ) );
-
-		return {
-			title: getEditedPostAttribute( 'title' ),
-			blocks: getBlocks(),
-			isTitleSupported: postType?.supports?.title ?? false,
-		};
-	} );
+	const { blocks, title, isTitleSupported, isShowingTemplate } = useSelect(
+		( select ) => {
+			const { getBlocks } = select( blockEditorStore );
+			const { getEditedPostAttribute, getRenderingMode } =
+				select( editorStore );
+			const { getPostType } = select( coreStore );
+			const postType = getPostType( getEditedPostAttribute( 'type' ) );
 
+			return {
+				title: getEditedPostAttribute( 'title' ),
+				blocks: getBlocks(),
+				isTitleSupported: postType?.supports?.title ?? false,
+				isShowingTemplate:
+					postType?.slug === 'wp_template' ||
+					getRenderingMode() === 'template-locked',
+			};
+		}
+	);
 	const prevHeadingLevelRef = useRef( 1 );
+	const outlineElements = computeOutlineElements( blocks, isShowingTemplate );
+	const headings = outlineElements.filter(
+		( item ) => item.type === 'heading'
+	);
+	const mainElements = outlineElements.filter(
+		( item ) => item.type === 'main'
+	);
+	const headingsByLevel = headings.reduce( ( acc, heading ) => {
+		acc[ heading.level ] = ( acc[ heading.level ] || 0 ) + 1;
+		return acc;
+	}, {} );
+	const hasMultipleH1 = headingsByLevel[ 1 ] > 1;
+
+	/**
+	 * TODO: Update hasTitle to work with the iframe in the block editor.
+	 * The titleNode is not found with the querySelector when the editor is iframed.
+	 * See https://github.com/WordPress/gutenberg/issues/47624
+	 */
+	const titleNode = document.querySelector( '.editor-post-title__input' );
+	const hasTitle = isTitleSupported && title && titleNode;
 
-	const headings = computeOutlineHeadings( blocks );
-	if ( headings.length < 1 ) {
-		return (
-			<div className="editor-document-outline has-no-headings">
-				<EmptyOutlineIllustration />
+	return (
+		<div
+			className={ clsx(
+				'document-outline',
+				headings.length < 1 && 'has-no-headings',
+				isShowingTemplate && mainElements.length === 0 && 'has-no-main'
+			) }
+		>
+			{ headings.length < 1 && (
+				<>
+					<EmptyOutlineIllustration />
+					<p>
+						{ __(
+							'Navigate the structure of your document and address issues like empty or incorrect heading levels.'
+						) }
+					</p>
+				</>
+			) }
+			{ isShowingTemplate && mainElements.length === 0 && (
 				<p>
 					{ __(
-						'Navigate the structure of your document and address issues like empty or incorrect heading levels.'
+						'The main element is missing. Select the block that contains your most important content and add the main HTML element in the Advanced panel.'
 					) }
 				</p>
-			</div>
-		);
-	}
+			) }
+			<ul>
+				{ outlineElements.map( ( item ) => {
+					const isHeading = item.type === 'heading';
+					const isMain = item.type === 'main';
+					const level = isMain ? __( 'Main' ) : `H${ item.level }`;
 
-	// Not great but it's the simplest way to locate the title right now.
-	const titleNode = document.querySelector( '.editor-post-title__input' );
-	const hasTitle = isTitleSupported && title && titleNode;
-	const countByLevel = headings.reduce(
-		( acc, heading ) => ( {
-			...acc,
-			[ heading.level ]: ( acc[ heading.level ] || 0 ) + 1,
-		} ),
-		{}
-	);
-	const hasMultipleH1 = countByLevel[ 1 ] > 1;
+					const isValid = isMain
+						? mainElements.length === 1
+						: ! item.isEmpty &&
+						  !! item.level &&
+						  item.level <= prevHeadingLevelRef.current + 1 &&
+						  ( item.level !== 1 ||
+								( ! hasMultipleH1 && ! hasTitle ) );
 
-	return (
-		<div className="document-outline">
-			<ul>
-				{ hasTitle && (
-					<DocumentOutlineItem
-						level={ __( 'Title' ) }
-						isValid
-						onSelect={ onSelect }
-						href={ `#${ titleNode.id }` }
-						isDisabled={ hasOutlineItemsDisabled }
-					>
-						{ title }
-					</DocumentOutlineItem>
-				) }
-				{ headings.map( ( item ) => {
-					// Headings remain the same, go up by one, or down by any amount.
-					// Otherwise there are missing levels.
-					const isIncorrectLevel =
-						item.level > prevHeadingLevelRef.current + 1;
-
-					const isValid =
-						! item.isEmpty &&
-						! isIncorrectLevel &&
-						!! item.level &&
-						( item.level !== 1 ||
-							( ! hasMultipleH1 && ! hasTitle ) );
-					prevHeadingLevelRef.current = item.level;
-
-					return (
-						<DocumentOutlineItem
-							key={ item.clientId }
-							level={ `H${ item.level }` }
-							isValid={ isValid }
-							isDisabled={ hasOutlineItemsDisabled }
-							href={ `#block-${ item.clientId }` }
-							onSelect={ () => {
-								selectBlock( item.clientId );
-								onSelect?.();
-							} }
-						>
-							{ item.isEmpty
-								? emptyHeadingContent
-								: getTextContent(
-										create( {
-											html: item.attributes.content,
-										} )
-								  ) }
-							{ isIncorrectLevel && incorrectLevelContent }
-							{ item.level === 1 &&
-								hasMultipleH1 &&
-								multipleH1Headings }
-							{ hasTitle &&
-								item.level === 1 &&
-								! hasMultipleH1 &&
-								singleH1Headings }
-						</DocumentOutlineItem>
-					);
+					if ( isShowingTemplate && isMain ) {
+						return (
+							<DocumentOutlineItem
+								key={ item.clientId }
+								level={ level }
+								isValid={ isValid }
+								isDisabled={ hasOutlineItemsDisabled }
+								href={ `#block-${ item.clientId }` }
+								onSelect={ () => {
+									selectBlock( item.clientId );
+									onSelect?.();
+								} }
+							>
+								{ ! isValid && incorrectMainTag }
+							</DocumentOutlineItem>
+						);
+					}
+
+					if ( isHeading ) {
+						// Headings remain the same, go up by one, or down by any amount.
+						// Otherwise there are missing levels.
+						const isIncorrectLevel =
+							item.level > prevHeadingLevelRef.current + 1;
+						prevHeadingLevelRef.current = item.level;
+
+						return (
+							<>
+								<DocumentOutlineItem
+									key={ item.clientId }
+									level={ level }
+									isValid={ isValid }
+									isDisabled={ hasOutlineItemsDisabled }
+									href={ `#block-${ item.clientId }` }
+									onSelect={ () => {
+										selectBlock( item.clientId );
+										onSelect?.();
+									} }
+								>
+									{ item.isEmpty
+										? emptyHeadingContent
+										: getTextContent(
+												create( {
+													html: item.attributes
+														.content,
+												} )
+										  ) }
+									{ isIncorrectLevel &&
+										incorrectLevelContent }
+									{ item.level === 1 &&
+										hasMultipleH1 &&
+										multipleH1Headings }
+									{ hasTitle &&
+										item.level === 1 &&
+										! hasMultipleH1 &&
+										singleH1Headings }
+								</DocumentOutlineItem>
+							</>
+						);
+					}
+					return null;
 				} ) }
 			</ul>
 		</div>
diff --git a/packages/editor/src/components/document-outline/style.scss b/packages/editor/src/components/document-outline/style.scss
index 49ce0c9b2d132..e922d817cb56e 100644
--- a/packages/editor/src/components/document-outline/style.scss
+++ b/packages/editor/src/components/document-outline/style.scss
@@ -82,7 +82,8 @@
 	padding: 1px 0;
 }
 
-.editor-document-outline.has-no-headings {
+.document-outline.has-no-headings,
+.document-outline.has-no-main {
 	& > svg {
 		margin-top: $grid-unit-30 + $grid-unit-05;
 	}