Sophomore Travis Souza's three-pointer with 2:45 remaining pulled the Anteaters (17-14, 10-7 in conference) within 59-58, but the Tigers' Tony Gill scored back-to-back baskets underneath to extend Pacific's lead to 63-58 with 1:52 left.
Freshman Alex Young scored on a driving layup to cut the margin to 63-60 with 1:42 remaining before Travis Fulton hit a pair of free throws with 1:25 left for a 65-60 Pacific lead.
The Tigers (18-12, 12-5) hit five of six free throws in the final 36 seconds to pull out the victory.
Young scored 13 points, all in the second half, and added seven rebounds to lead the Anteaters. Sophomore Will Davis II scored 12 points and recorded six boards and three blocked shots for the 'Eaters, who had their five-game winning streak snapped.
Senior Daman Starring scored eight points for UCI and has 1,000 in his Anteater career, becoming the 22nd member of that club. Senior Mike Wilder and redshirt sophomore John Ryan also scored eight points as Ryan tied his career high.
Young, Starring and Wilder each led the 'Eaters with three assists.
Senior Adam Folker, who is tied with Davis as the team's leading rebounder and is UCI's second-leading scorer, did not travel with the team, UCI head coach Russell Turner said.
This was the last Big West Conference game between the schools, as Pacific is moving to the West Coast Conference July 1.
"This was like two different games for us," Turner said. "I am really proud of how our team came back. We got so close, but had some fouls and missed some things because I think we were so excited to have come back so far.
"I am very proud of these guys."
UCI closes out the regular season Saturday night at UC Davis with the final seeding for next week's Big West Tournament at the Honda Center to be determined following Saturday's games.
Long Beach State (14-3 in league) clinched the regular-season title Thursday night. Pacific (12-5) is in second place, Cal Poly (11-6) third, UCI (10-7) fourth, Hawai'i (10-8) fifth, UC Davis (9-8) sixth, and Cal State Fullerton (6-11) and UC Santa Barbara (6-11) are tied for seventh.
BYU 6, UC Irvine 3
PROVO, Utah — A four-run seventh put the game out of reach for the No. 21-ranked Anteaters, who dropped their second series opener Thursday.
Right fielder Kris Paulino and designater hitter Ryan Cooper both went two for three for UCI (11-3).
Anteaters' starter Andrew Thurman pitched 6 2/3 innings, allowing single runs in the fourth, sixth and the seventh to fall to 1-2 on the year. The right-hander struck out three and walked one in the effort.
Counterpart Desmond Poulson held UCI to two runs in 6 2/3 innings, striking out five and walking three but did not register a decision in the duel.