Morrow, who finished with 91 yards rushing and 47 receiving, took a shovel pass from quarterback Luke Falk 23 yards for a touchdown that put Washington State (5-0, 2-0 Pac-12 Conference) ahead 27-20 in the fourth quarter. USC responded when Trojans quarterback Sam Darnold rushed for his second touchdown of the night, capping a drive that freshman wide receiver Tyler Vaughns extended with an acrobatic, fourth-down catch.
But Morrow delivered again on the ensuing Washington State possession, his carries of 11 and 35 yards moving the Cougars into USC territory. Erik Powell's 32-yard field goal proved to be the game-winner, as USC's last-gasp drive ended with Jahad Woods forcing a Darnold fumble.
Washington State matched the final score of its last win over USC (4-1, 2-1) at Martin Stadium. Jason Gesser outdueled Heisman Trophy winner Carson Palmer in that meeting in 2002.
Washington State's ability to convert on third down, and USC's inability to do the same, shaped the course of the game. The Cougars went 8-of-18 on third downs; USC went 2-of-11.
Falk also outperformed Darnold statistically to set the tone for Washington State. The Cougars quarterback finished with 340 yards passing and two touchdowns. The shovel pass score to Morrow tied Falk with Marcus Mariota as the Pac-12's No. 2 all-time career leader for passing touchdowns.
Morrow's goal-line touchdown run pushed Washington State into a 17-17 tie at halftime. Morrow's score ended a 12-play, 94-yard drive.
USC running back Ronald Jones II returned from an ankle injury that sidelined him a week ago at Cal to rush for 118 of his 128 yards in the first half, including 86 on a touchdown rush in the second quarter.
Jones' long scoring run answered a Washington State score set up by the Cougars' own explosive play.
Falk found wide receiver Renard Bell open after a USC coverage breakdown, hitting Bell in stride for a 61-yard gain. That led to a 28-yard score on a swing pass from Falk to Tavares Martin Jr.
The touchdown gave Washington State its second brief lead of the first half. The Cougars struck first when a promising, game-opening drive stalled, ending in a 44-yard Powell field goal.
Darnold capped USC's opening drive -- 10 plays, 75 yards -- with a 4-yard touchdown rush. The Trojans' next three possessions produced a single first down and ended with two punts and an interception when Sean Harper Jr. undercut a route.
The Washington State defense again came through when an Uchenna Nwosu interception of Falk set the Trojans up inside the Cougars' 5-yard line. Washington State snuffed out three USC plays, holding the Trojans to a 20-yard Chase McGrath field goal.
NOTES: USC, which came in without LT Toa Lobendahn, lost RT Chuma Edoga and G Viane Talamaivao to injury in the first half ... QB Luke Falk's 61-yard pass completion to WR Renard Bell was both Washington State's longest play of the season to date, and the longest play the USC defense allowed. ... USC QB Sam Darnold's eight interceptions this season are one fewer than the nine he threw in the entire 2016 campaign. ... With his 86-yard touchdown rush, USC RB Ronald Jones II has at least one score in 11 consecutive games in which he's suited up dating to last season.