The rookie guard returned to Memphis' lineup and scored 26 points to lead the Grizzlies to a 110-102 victory Monday night.
Morant added seven assists, two rebounds and a steal in 29 minutes after missing the previous four games with a sore back.
The No. 2 pick in the draft came up particularly big for Memphis (7-16) in the fourth quarter, scoring 13 points as the Warriors tried to claw their way back into it.
"There's a lot of trust with the ball in his hands to make the right play," Grizzlies coach Taylor Jenkins said. "He had a couple assists late that we needed. Big moments for him, and it's awesome to see so early in his career."
The Warriors (5-20) finished a five-game road trip with a victory in Chicago on Friday and were trying to win consecutive games for the first time this season.
Despite long-term injuries to stars Stephen Curry and Klay Thompson, their roster is healthier and the bench deeper than it's been in weeks.
But Golden State shot just 38% from the floor and looked out of sync on both ends of the court for much of the night.
"I honestly think we just (messed) around with the game," forward Draymond Green said. "We're not good enough to do that. Our transition defense was bad. ... We played pretty horrible the entire game. It was definitely disappointing."
Dillon Brooks scored 17 points and Jaren Jackson Jr. had 16 for the Grizzlies, who won for just the second time in 11 games.
D'Angelo Russell and Alec Burks each scored 18 for the Warriors, but Russell shot just 6 of 22.
Morant, who entered the night leading NBA rookies at 18.6 points per game, was one of several Grizzlies who had an answer every time Golden State tried to make a run.
With just over a minute remaining and the Warriors within 106-100, Morant drove the middle of the lane and went flying for a monster dunk that hushed the Chase Center crowd.
Grizzlies: Brandon Clarke, who has been bothered by left hip and left oblique soreness, missed his fourth game in a row. ... Kyle Anderson didn't play after testing out his sore right heel in warmups.
Warriors: Jacob Evans III, sidelined since Oct. 28 with a left hip strain, played nine minutes in his return Monday. ... Damion Lee, out with a fractured right hand, originally was listed as probable, but coach Steve Kerr said he needs more time before returning to the active roster.
Grayson Allen scored 15 points for Memphis and hit a couple timely shots from beyond the arc when the Warriors tried to rally. But Allen left the game in the third quarter after twisting his right ankle.
"Hopefully he's back pretty soon," Jenkins said.
KERR ON GUN CONTROL
On Monday night, the Warriors hosted a group of first responders and others affected by a mass shooting in Gilroy, California in July that left four dead and many more injured.
Kerr, who has been outspoken about gun violence, was asked about the issue before the game and reiterated his feelings about the need for background checks before someone purchases a gun.
"The first thing that has to happen is our government officials have to decide that the safety of our citizens is more important than the money they receive from the NRA (National Rifle Association)," Kerr said.
Grizzlies: Visit Phoenix on Wednesday to conclude their four-game road trip.
Warriors: Wednesday they host the New York Knicks, the only team in the NBA that entered Monday with a worse record than Golden State.
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Morant scores 26 points in return vs. Warriors
Ja Morant leads all scorers with 26 points in his return to lift the Grizzlies over the Warriors.