Dave Nwaba and Joel Awich each chipped in 11 points for Cal Poly (11-19), which will have to wait for the other three quarterfinal games to finish to determine its next foe.
Michael Bryson scored 19 points and Alan Williams tallied 14 points and 10 rebounds for UCSB (21-9).
The Mustangs were hot out of the gate, making 59.3 percent from the floor, including 6-of-8 from 3-point range, in the first half. The Gauchos had only eight field goals on 33.3 percent shooting in the frame and trailed 41-20 at the break.
Although Cal Poly dropped to 25 percent shooting in the second half, UCSB could not capitalize, shooting just 32 percent itself, while being outscored 14-0 at the free-throw line.
UCSB did not get a single point from the charity stripe, while Cal Poly had 17.