diff --git a/client/blocks/reader-full-post/header-meta.jsx b/client/blocks/reader-full-post/header-meta.jsx
index bfd5b257b6dd..784d6d7bcf3d 100644
--- a/client/blocks/reader-full-post/header-meta.jsx
+++ b/client/blocks/reader-full-post/header-meta.jsx
@@ -1,25 +1,29 @@
 import { TimeSince } from '@automattic/components';
 import PropTypes from 'prop-types';
+import { useSelector } from 'react-redux';
 import ReaderAuthorLink from 'calypso/blocks/reader-author-link';
 import ReaderSiteStreamLink from 'calypso/blocks/reader-site-stream-link';
 import UserAvatar from 'calypso/blocks/user-avatar';
 import { areEqualIgnoringWhitespaceAndCase } from 'calypso/lib/string';
 import { getStreamUrl } from 'calypso/reader/route';
+import { getFeed } from 'calypso/state/reader/feeds/selectors';
 
 const ReaderFullPostHeaderMeta = ( { post, author, siteName, feedId, siteId } ) => {
 	const streamUrl = getStreamUrl( feedId, siteId );
-
+	const feed = useSelector( ( state ) => getFeed( state, feedId ) );
 	const hasAuthorName = author?.name;
 	const hasMatchingAuthorAndSiteNames =
 		hasAuthorName &&
 		areEqualIgnoringWhitespaceAndCase( String( siteName ), String( author?.name ) );
 	const showAuthorLink = hasAuthorName && ! hasMatchingAuthorAndSiteNames;
+	const avatarUrl =
+		! author.avatar_URL && post.is_external ? feed?.site_icon || feed?.image : author?.avatar_URL;
 
 	return (
 		<div className="reader-full-post__header-meta-wrapper">
 			<UserAvatar
 				className="reader-full-post__header-meta-avatars"
-				user={ author }
+				user={ { ...author, avatar_URL: avatarUrl } }
 				iconSize={ 40 }
 			/>
 			<div className="reader-full-post__header-meta-info">
diff --git a/client/blocks/reader-full-post/test/header-meta.jsx b/client/blocks/reader-full-post/test/header-meta.jsx
new file mode 100644
index 000000000000..cfb5339fba92
--- /dev/null
+++ b/client/blocks/reader-full-post/test/header-meta.jsx
@@ -0,0 +1,91 @@
+/**
+ * @jest-environment jsdom
+ */
+import { render } from '@testing-library/react';
+import { Provider } from 'react-redux';
+import { createStore } from 'redux';
+import UserAvatar from 'calypso/blocks/user-avatar';
+import { getFeed } from 'calypso/state/reader/feeds/selectors';
+import ReaderFullPostHeaderMeta from '../header-meta';
+
+jest.mock( '@automattic/components', () => ( {
+	TimeSince: () => null,
+} ) );
+
+jest.mock( 'calypso/blocks/user-avatar', () => jest.fn( () => null ) );
+
+jest.mock( 'calypso/state/reader/feeds/selectors', () => ( {
+	getFeed: jest.fn(),
+} ) );
+
+const createMockStore = () => {
+	const reducer = ( state = {} ) => state;
+	return createStore( reducer );
+};
+
+const renderHeaderMeta = ( { post, author, siteName, feedId, siteId } = {} ) => {
+	const store = createMockStore();
+	return render(
+		<Provider store={ store }>
+			<ReaderFullPostHeaderMeta
+				post={ post }
+				author={ author }
+				siteName={ siteName }
+				feedId={ feedId }
+				siteId={ siteId }
+			/>
+		</Provider>
+	);
+};
+
+describe( 'ReaderFullPostHeaderMeta avatar fallback', () => {
+	const testAuthor = { name: 'Test Author', avatar_URL: 'https://example.com/author-avatar.png' };
+
+	beforeEach( () => {
+		jest.clearAllMocks();
+		getFeed.mockReturnValue( undefined );
+	} );
+
+	it( 'keeps author avatar when one already exists', () => {
+		getFeed.mockReturnValue( {
+			site_icon: 'https://example.com/site-icon.png',
+		} );
+
+		renderHeaderMeta( {
+			author: testAuthor,
+			post: { is_external: true },
+		} );
+
+		const avatarProps = UserAvatar.mock.calls[ 0 ][ 0 ];
+		expect( avatarProps.user.avatar_URL ).toBe( 'https://example.com/author-avatar.png' );
+	} );
+
+	it( 'uses feed site_icon when author avatar is missing for external posts', () => {
+		getFeed.mockReturnValue( {
+			site_icon: 'https://example.com/site-icon.png',
+			image: 'https://example.com/feed-image.png',
+		} );
+
+		renderHeaderMeta( {
+			post: { is_external: true },
+			author: { ...testAuthor, avatar_URL: undefined },
+		} );
+
+		const avatarProps = UserAvatar.mock.calls[ 0 ][ 0 ];
+		expect( avatarProps.user.avatar_URL ).toBe( 'https://example.com/site-icon.png' );
+	} );
+
+	it( 'falls back to feed image when feed site_icon is unavailable', () => {
+		getFeed.mockReturnValue( {
+			image: 'https://example.com/feed-image.png',
+		} );
+
+		renderHeaderMeta( {
+			post: { is_external: true },
+			author: { ...testAuthor, avatar_URL: undefined },
+		} );
+
+		const avatarProps = UserAvatar.mock.calls[ 0 ][ 0 ];
+		expect( avatarProps.user.avatar_URL ).toBe( 'https://example.com/feed-image.png' );
+	} );
+} );