UT-Pan American's Dan Kimasa stole the ball as Grand Canyon tried to inbound, leading to Watson's winning 3-footer.
Ryan Majerle hit a 3-pointer, sparking an 11-5 run as Grand Canyon, which trailed most of the game, rallied in the final minutes to lead 72-70 on a DeWayne Russell layup with 12 seconds left. The Antelopes (11-10, 2-2) lost their second straight in Western Athletic Conference play after falling to New Mexico on Thursday.
Shaquille Hines and Shaquille Boga scored 18 each for Texas-Pan American (8-13, 2-3) and Janari Joesaar added 16 points with 11 rebounds. Watson finished with seven points and six assists - both career highs.
Joshua Braun scored a career-high 25 for the Antelopes, Russell added 15 and Majerle 13.