Strikeforce: Champions takes place January 12 from Oklahoma City.
Barnett (31-6) is 2-1 in Strikeforce, falling to Daniel Cormier earlier this year in the finals of the heavyweight grand prix. He is a former UFC champion, and had not lost in six years prior to facing Cormier.
Guelmino (11-3-1) is on a seven-fight win streak. He owns six submission victories and three more via knockout.
The card, which is rumored to be the last one for Strikeforce, has lost two championship matches in recent weeks. The headliner is now Nate Marquardt vs. Tarec Saffiedine for Marquardt’s welterweight title.