His driving layup with 3 seconds left gave the Billikens (26-5, 13-3 Atlantic 10) their second successive outright conference title and snapped a three-game losing skid after a school-record 19-game winning streak.
"We had guys make big plays throughout," Saint Louis coach Jim Crews said. "From big passes, to big post ups to big blocks to big rebounds to big shots. ... I'm really proud of our guys."
Jett finished with 17 points, including the Billikens' last six of the game. Saint Louis got its final possession on a jump ball call with 36.4 seconds left. Jett dribbled out the clock near midcourt before starting the game-winning drive.
"That's in our plays and [Crews] told me to make a play whether it's pass or shoot," Jett said.
Chaz Williams dribbled down and got off a long 3-point attempt before the buzzer that bounced off the rim. Williams had 20 points and nine assists for Massachusetts (23-7, 10-6).
Saint Louis traveled to Amherst knowing it had the No. 1 seed in the upcoming conference tournament wrapped up and with a share of the regular season championship. But the Billikens needed a victory to seal it outright for the second successive season.
"What they've done is extraordinary," Crews said. "Winning last year's regular season and winning the A-10 conference tournament and then backing it up with this year, right today, outright, is certainly a credit to some guys who have done some phenomenal things."
Jett's winning play ended the regular season on the high note and gave the locker room a much-needed boost of confidence after the skid.
"It definitely gives us momentum," Jett said. "We needed a win desperately to claim outright A-10 titles back-to-back, so we knew what it was coming into it and we came through."
Massachusetts could have locked up the No. 4 seed and a bye with a victory, but instead will be the No. 6 seeed and play No. 11 Rhode Island on Thursday.
"I didn't get a good enough running start," Williams said of his final shot. "I only had three seconds, that was my fault. I had to take the shot from further out. It's just one of those shots where you just have to make something happen."
Saint Louis trailed 35-33 at halftime and the Minutemen controlled the majority of the second half as the Billikens nipped at their heels.
Massachusetts took its largest lead at 57-49 as Derrick Gordon made two free throws with 5:48 to play.
"I felt like we were in control, but it was really only one or two possessions away," Minutemen coach Derek Kellogg said.
Saint Louis' Dwayne Evans tied the game at 56 with 3:16 left, but the Minutemen responded with two straight baskets.
Sampson Carter made it a four-point game, but Evans cut the lead to 60-58 with 2:14 left.
Coming out of a timeout, Carter missed a 3-pointer for what Kellogg thought was a crucial play with 1:33 remaining.
"When Sam missed that 3 in front of our bench. I knew we might be in a little bit of trouble," he said.
Jett was fouled and made both his free throws to being his closing six-point run.