Texas Tech scored the final 21 points in the game.
Harrell completed 41-of-50 passes for 309 yards for the Red Raiders (11-1, 7-1 Big 12), who clinched at least a share of the Big 12 South title.
Detron Lewis had 68 yards on nine receptions and Michael Crabtree also had nine, for 63 yards, before leaving in the second quarter with a right ankle injury. Baron Batch had two rushing scores for Texas Tech.
Robert Griffin ran for two touchdowns and passed for another for Baylor (4-8, 2-6), which lost five of its last six games. Griffin had 99 yards running and 91 passing.