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.