He faced fellow finalist Daniel Cormier on May 19, 2012 at Strikeforce: Barnett vs. Cormier to determine the Strikeforce Heavyweight Grand Prix Champion. Dana White announced if Barnett were to defeat Cormier, he would be allowed back into the UFC, "If he wins the fight, I can't see why he wouldn't come [to the UFC]." Barnett lost the bout via unanimous decision (49-46, 50-45, 50-45), losing for the first time in over 5 years. In January 2013, Barnett faced a promotional newcomer, Nandor Guelmino, at the final Strikeforce event. Barnett defeated Guelmino via arm-triangle choke submission in the first round.