Mainstage 2021 was a Super Smash Bros. Ultimate supermajor and Super Smash Bros. Melee major held in Ontario, California, from November 12th-14th, 2021. It was Beyond the Summit's second Smash tournament outside of the Smash Summit tournament series. The top 6 non-invited players for Melee and Ultimate qualified for Smash Summit 12 and Smash Ultimate Summit 4, respectively. The tournament also ran a Nickelodeon All-Star Brawl singles bracket. The tournament had the largest attendance of a SoCal Super Smash Bros. tournament, beating Red Bull Smash Gods and Gatekeepers.
Super Smash Bros. Melee singles
Super Smash Bros. Melee doubles
Super Smash Bros. Ultimate singles
*DQ'd in losers.
Super Smash Bros. Ultimate doubles