The loss drops Princeton to 6-7 on the season, while Siena won for just the second time in its last nine games to improve to 4-7.
Princeton led by as much as eight at 24-16 at the midway point of the first half, but Siena answered with a 10-1 run to take the lead before Princeton held a three-point advantage at the half.
The Tigers held that advantage and built it to six at 43-37 with seven minutes to play before Siena got hot again, scoring 10 straight points to take the lead for good.
Princeton had been 6-0 this season when at least three players reached double figures, as T.J. Bray, Douglas Davis and Ian Hummer did against the Saints. Bray had a career-high 15 points, while Hummer added 15 and Davis led all scorers with 23, his most since a loss to Presbyterian (also 23) last November.
Outside of Hummer, Bray and Davis, however, the other six Tigers to see court time scored only five points. Davis canned seven 3-pointers, tying a career best set two years ago against Lafayette. With the loss, however, that was little reason for pride.
"It's tough to lose any game, whether you're making shots or missing shots," Davis said. "Every loss hurts."
With 31 3-pointers tried as a team, Princeton hoisted up 30-plus from beyond the arc for just the third time this season and first since the weekend at Bucknell at the end of November.
"It's frustrating for us," Mitch Henderson, the Franklin C. Cappon-Edward G. Green '40 head coach of Princeton men's basketball, said. "I thought 31 threes was a lot, but they were coming from guys that we like shooting the ball. It just seemed though that we settled a little bit for threes versus the zone."
Evan Hymes led Siena with 22 points while the two other Saints in double figures each had a double-double. Hymes' fellow guyard Kyle Downey had 15 points and 10 rebounds, while MAAC rebounding leader OD Anosike scored 12 points with 10 boards. Even with the two double-doubles, Siena won the rebounding battle just 36-34.
"I really felt like the key to the game was Evan Hymes. We had no answer for him and he seemed to do everything for them," Henderson said. "We just couldn't stop him."
Bray, who set a career best with seven assists, concurred.
"He's quick. He's lightning fast," Bray said. "There were times when we shied away from the scouting report and that was costly for us. He ended up with 22 points. He's a very good player and will be for a couple more years (here)."
Hymes was 7 of 8 from the free-throw line while Downey was 7 of 10 as Siena enjoyed a 27-6 edge in tries from the stripe.
"It just felt like there wasn't enough balance from our end," Henderson said. "We could get the ball inside when we wanted, we just didn't do it."