Ricky Harris scored 30 points and Gary Forbes added 24 for Massachusetts (13-5, 2-2), which had its two-game winning streak snapped. Dante Milligan had 12 points and 10 rebounds.
Harris hit a 3-pointer early in the second half that cut Saint Joseph's lead to 30-19. Four Hawks scored in a 9-0 run and Saint Joseph's extended its lead to 39-19 with 14:58 remaining.
Massachusetts got within 47-39 with 9:03 left, but the Hawks scored eight straight points to take a commanding 55-39 lead.
Massachusetts chipped away and closed within 77-71 with 25 seconds left on a steal and layup by Harris. A 3-pointer by Forbes with 3.5 seconds remaining closed the gap to 79-77. But Calathes hit two free throws to seal the victory.
The Minutemen shot 40 percent from the field and 8-for-26 from 3-point range. Saint Joseph's was 39-of-54 from the free throw line, while the Minutemen went 9-for-18.
Saint Joseph's led 29-16 at halftime. Both teams struggled from the field. The Hawks shot 28 percent (9-of-32), while the Minutemen shot 20 percent (7-of-35).
Massachusetts also misfired on all 10 of its 3-pointers and was outrebounded 29-24.
It was the lowest-scoring half for Massachusetts this season and the team's fewest since a nine-point effort at Temple on Feb. 1, 2006. In that game, the Minutemen trailed 34-9 at halftime and wound up losing 76-47.
