It almost stung for the Gaels.
BYU's Tyler Haws hit a jumper in the paint with 2.5 seconds left to give the Cougars a 69-67 lead, but Dellavedova found some space BYU was a bit slow getting back and drained perhaps the biggest shot of his stellar career.
"I'm just kinda stunned," Haws said. "I am at a loss for words. I thought we had the game won, and Dellavedova hit a big shot. Gotta give him credit."
Saint Mary's coach Randy Bennett said the Gaels did everything right after Haws' heavily defended basket, but acknowledged a little bit of luck helped as well.
"We got a little lucky, and the shot went in," Bennett said.
BYU fell to 4-1 in WCC play, 14-5 overall, in falling to SMC for the third straight time. The Gaels improved to 3-1 and 14-4.
The Cougars led by 16 points in the first half and 34-27 at halftime, but couldn't hold the lead.
Stephen Holt led SMC with 22 points and Dellavedova added 18; Haws had 23 to lead BYU.
Just as Santa Clara did on Saturday, SMC quickly erased BYU's halftime lead with a 14-4 run.
The Cougars looked like they were in trouble when Brandon Davies (12 points, no rebounds) picked up his fourth foul with 12 minutes left, but they rallied with their star on the bench.
Brock Zylstra's 3-pointer with 6:27 left gave the Cougars a 54-53 lead, and the lead see-sawed the remainder of the way.
Haws' fadeaway jumper with 40 seconds left gave BYU a 67-65 lead, but Dellavedova answered with a driving basket, running over Nate Austin in the process, to tie it with 34 seconds left.
The Cougars worked the clock to single digits before Haws hit what many believed was the game-winner, only to watch Dellavedova's desperation heave go in.
"Yeah, this one hurts," Haws said.
Matt Carlino, who almost didn't play due to back spasms, scored 16 points for BYU, 10 in the first half.
"It is always hard to lose a game like that," Carlino said. "He hit a heckuva shot."
Rose said the Cougars lost the big lead because they didn't consistently score in the first half, and because SMC presented some difficult defensive matchups. He said Dellavedova got the shot off in time, and it was on line all the way. The only question was whether it would be long, short or in.
"Just a great college basketball game," Rose said. "Two teams battling really hard and players making plays. ... It came down to a guy making a really tough play late in the game."
O A 35-foot buzzer-beater by Matthew Dellavedova seals the win for Saint Mary's.
• Tyler Haws has 23 points to lead the Cougars, including a shot with 2.5 seconds left that put them up 69-67.
More coverage • BYU coach Dave Rose misses his chance at a milestone. > D4
Kurt Kragthorpe • Two months from now, BYU will remember the last two seconds of Wednesday's loss. > sltrib.com/sports
Saint Mary's 70, BYU 69
SAINT MARY'S (14-4) • Waldow 1-3 2-2 4, Page 0-2 0-0 0, Young 5-8 0-0 10, Dellavedova 6-16 3-4 18, Holt 8-16 1-3 22, Rowley 0-0 0-0 0, Giusti 0-3 0-0 0, Levesque 0-2 1-2 1, Hodgson 0-1 2-2 2, Walker III 5-9 1-2 13. Totals • 25-60 10-15 70.
BYU (14-5) • Davies 6-14 0-0 12, Haws 8-16 7-8 23, Carlino 6-12 2-3 16, Sharp 1-1 0-0 3, Zylstra 2-2 0-0 6, Cusick 0-0 0-0 0, Ambrosino 0-1 0-0 0, Winder 1-1 0-0 3, Austin 3-4 0-1 6. Totals 27-51 9-12 69. HalftimeBYU 34-27. 3-Point GoalsSaint Mary's (Cal) 10-25 (Holt 5-9, Dellavedova 3-8, Walker III 2-5, Giusti 0-1, Page 0-2), BYU 6-13 (Zylstra 2-2, Carlino 2-4, Sharp 1-1, Winder 1-1, Davies 0-1, Ambrosino 0-1, Austin 0-1, Haws 0-2). Fouled OutDavies. ReboundsSaint Mary's (Cal) 36 (Waldow, Walker III, Young 7), BYU 29 (Haws 7). AssistsSaint Mary's (Cal) 15 (Dellavedova 8), BYU 18 (Carlino 5). Total FoulsSaint Mary's (Cal) 17, BYU 14. A14,857.