NBA's Ben Simmons Competing in FaZe Clan's Call of Duty Tourney On Twitch Rivals

It may be the offseason for Ben Simmons, but the NBA superstar is getting right back into competition mode ... taking part in a HUGE Call of Duty tournament backed by FaZe Clan.

TMZ Sports has learned ... Simmons -- who recently invested in FaZe Clan -- is part of a celeb-packed lineup for the "Twitch Rivals: FaZe Face-Off Ft. Warzone" event going down on September 17.

Simmons -- AKA "FaZe Simmo" will join big names like Marshmello, Slim Jxmmi, Carnage, Lil Durk, Bulls star Zach LaVine and USWNT Allie Long in the Call of Duty competition, which is the first event from FaZe Clan's new series with the Twitch Rivals streaming service.

The competition will also feature FaZe Clan regulars like FaZe Apex, FaZe Rug, NICKMERCS, FaZe Temperrr, and FaZe SWAGG ... as well as a bunch of other well-known streamers.

The event will consist of up to 20 teams of 4 players ... who will compete for a prize pool of $75,000!

It's been a big year for FaZe Clan ... the org. just wrapped up tournament for the game Valorant -- which was viewed online more than 5.2 MILLION times!

Oh yeah, and Bronny James just joined the squad!!!