The top-ranked Bruins (29-3) won their ninth title in men's water polo and their first since 2004. USC (25-7) played in its 10th consecutive championship match.
Gordon Marshall ended an 8-8 tie by scoring with 34 seconds left.
UCLA junior Danny McClintick led all scorers with four goals in the title match and was selected the most valuable player of the tournament.
Kostas Genidounias and Marc Vonderweidt had two goals each for the third-ranked Trojans, who began the fourth quarter trailing, 7-4. The Bruins led, 4-2, at halftime.
UCLA goalie Garrett Danner made eight saves and USC's McQuin Baron made six.