The 2021 Extreme Rules was the 13th annual Extreme Rules professional wrestling pay-per-view (PPV) and livestreaming event produced by WWE. It was held for wrestlers from the promotion's Raw and SmackDown brand divisions. The event took place on September 26, 2021, at the Nationwide Arena in Columbus, Ohio, and was the first Extreme Rules to livestream on Peacock. It was also the first and so far only Extreme Rules event to be held in September, replacing Clash of Champions that was previously held during that month; Extreme Rules was moved to October the following year.

The event was originally scheduled to take place on July 18 at the SAP Center in San Jose, California, but it was postponed and relocated due to issues stemming from the COVID-19 pandemic in 2020. Although the concept of Extreme Rules is that the event features various hardcore-based matches, for the 2021 event, only the main event match was contested under a hardcore stipulation. The 2021 event also saw a return to its original name, after the previous year's event was titled as The Horror Show at Extreme Rules. Notably, this event was also the first Extreme Rules event where no titles changed hands—five championships were defended and retained.

Seven matches were contested at the event, including one on the Kickoff pre-show. In the main event, Roman Reigns defeated "The Demon" Finn Bálor in an Extreme Rules match to retain SmackDown's Universal Championship. In other prominent matches, The Usos (Jey Uso and Jimmy Uso) defeated The Street Profits (Angelo Dawkins and Montez Ford) to retain the SmackDown Tag Team Championship, SmackDown Women's Champion Becky Lynch faced Bianca Belair, which ended in a no contest after Sasha Banks made her return and subsequently ambushed both competitors, and Damian Priest defeated Jeff Hardy and Sheamus in a triple threat match to retain Raw's United States Championship.

The event received mixed-to-negative reviews from critics and fans, who mainly criticized the lack of stipulation matches. Special criticism was reserved for the ending of the main event.