W3C Feed Validation Service, for Atom and RSS

Sitelet https://web.archive.org/web/20201103065156/http://validator.w3.org/feed/

Validate by URI
Validate by Direct Input

Validate by URI

Validate by direct input

This is the W3C Feed Validation Service, a free service that checks the syntax of Atom or RSS feeds. The Markup Validation Service is also available if you wish to validate regular Web pages.