diff --git a/packages/editor/src/components/document-outline/index.js b/packages/editor/src/components/document-outline/index.js
index 6c498ccc79990..188c6b8c88776 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 +78,50 @@ 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} isEditingTemplate Indicates if a template is being edited.
  *
- * @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 = [], isEditingTemplate = false ) => {
 	return blocks.flatMap( ( block = {} ) => {
-		if ( block.name === 'core/heading' ) {
+		const isHeading = block.name === 'core/heading';
+		const isMain =
+			isEditingTemplate && block.attributes?.tagName === 'main';
+
+		if ( isHeading ) {
 			return {
 				...block,
+				type: 'heading',
 				level: block.attributes.level,
 				isEmpty: isEmptyHeading( block ),
 			};
 		}
-		return computeOutlineHeadings( block.innerBlocks );
+
+		if ( isMain ) {
+			return {
+				...block,
+				type: 'main',
+				children: computeOutlineElements(
+					block.innerBlocks,
+					isEditingTemplate
+				),
+			};
+		}
+
+		return computeOutlineElements( block.innerBlocks, isEditingTemplate );
 	} );
 };
 
@@ -113,105 +143,182 @@ 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, isEditingTemplate } = 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,
+				isEditingTemplate: postType?.slug === 'wp_template',
+			};
+		}
+	);
 	const prevHeadingLevelRef = useRef( 1 );
 
-	const headings = computeOutlineHeadings( blocks );
-	if ( headings.length < 1 ) {
-		return (
-			<div className="editor-document-outline has-no-headings">
-				<EmptyOutlineIllustration />
-				<p>
-					{ __(
-						'Navigate the structure of your document and address issues like empty or incorrect heading levels.'
-					) }
-				</p>
-			</div>
-		);
-	}
+	const outlineElements = computeOutlineElements( blocks, isEditingTemplate );
+	const headings = outlineElements.filter(
+		( item ) => item.type === 'heading'
+	);
+	const mainElements = outlineElements.filter(
+		( item ) => item.type === 'main'
+	);
 
 	// 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,
-		} ),
-		{}
-	);
+
+	// Count the number of headings and nested headings by level, to determine if there are multiple H1s.
+	const countByLevel = outlineElements.reduce( ( acc, element ) => {
+		if ( element.type === 'heading' ) {
+			acc[ element.level ] = ( acc[ element.level ] || 0 ) + 1;
+		}
+		if (
+			element.type === 'main' &&
+			element.children &&
+			element.children.length > 0
+		) {
+			element.children.forEach( ( child ) => {
+				if ( child.type === 'heading' ) {
+					acc[ child.level ] = ( acc[ child.level ] || 0 ) + 1;
+				}
+			} );
+		}
+
+		return acc;
+	}, {} );
 	const hasMultipleH1 = countByLevel[ 1 ] > 1;
 
-	return (
-		<div className="document-outline">
-			<ul>
-				{ hasTitle && (
+	const documentOutlineItems = ( { item } ) => {
+		if ( isEditingTemplate && item.type === 'main' ) {
+			return (
+				<li key={ item.clientId }>
 					<DocumentOutlineItem
-						level={ __( 'Title' ) }
+						level={ __( 'Main' ) }
 						isValid
-						onSelect={ onSelect }
-						href={ `#${ titleNode.id }` }
 						isDisabled={ hasOutlineItemsDisabled }
+						href={ `#block-${ item.clientId }` }
+						onSelect={ () => {
+							selectBlock( item.clientId );
+							onSelect?.();
+						} }
 					>
-						{ title }
+						{ incorrectMainTag }
 					</DocumentOutlineItem>
+					{ /*
+					 * Some blocks with the main tagName, such as groups,
+					 * can have multiple levels of nested inner blocks.
+					 * These inner blocks should be included in the outline.
+					 */ }
+					{ item.children.length > 0 && (
+						<ul>
+							{ item.children.map( ( child ) =>
+								documentOutlineItems( {
+									item: child,
+									prevHeadingLevelRef,
+									hasMultipleH1,
+									hasTitle,
+									hasOutlineItemsDisabled,
+									selectBlock,
+									onSelect,
+								} )
+							) }
+						</ul>
+					) }
+				</li>
+			);
+		}
+
+		if ( item.type === 'heading' ) {
+			// 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 (
+				<li key={ item.clientId }>
+					<DocumentOutlineItem
+						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>
+				</li>
+			);
+		}
+
+		return null;
+	};
+
+	return (
+		<div
+			className={ clsx(
+				'document-outline',
+				headings.length < 1 && 'has-no-headings',
+				isEditingTemplate && 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>
+				</>
+			) }
+			{ isEditingTemplate && mainElements.length === 0 && (
+				<p>
+					{ __(
+						'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>
+			) }
+			<ul>
+				{ outlineElements.map( ( item ) =>
+					documentOutlineItems( {
+						item,
+						prevHeadingLevelRef,
+						hasMultipleH1,
+						hasTitle,
+						hasOutlineItemsDisabled,
+						selectBlock,
+						onSelect,
+					} )
 				) }
-				{ 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>
-					);
-				} ) }
 			</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;
 	}