Eric Atkins scored 17 points to help the Fighting Irish (19-8, 11-3 Big East) win their eighth straight game. The Irish made nine of their 12 3-pointers after halftime to send Villanova to its fifth loss in six games.
The Wildcats (11-15, 4-10) got a second chance after squandering the massive lead on Maurice Sutton's tying bucket late in regulation. They didn't have enough in OT.
Jerian Grant put the Irish ahead 68-65 on a 3-pointer with 1:28 left and Connaughton hit his seventh 3 with 45.9 seconds to go for a six-point lead.
The Irish survived 38-percent shooting from the floor for one of their toughest wins of the season.
JayVaughn Pinkston scored 24 points, Dom Cheek had 19, and Sutton had 12 points and 10 rebounds for the Wildcats. Leading scorer Maalik Wayns sat out with a sprained MCL in his left knee and guard James Bell was out with a sprained left ankle.
Down two starters, the Wildcats were at their best in the first half and led 39-19.
Not for long.
The Irish stormed back against a Villanova team that made only five baskets in the second half. The fifth one was a big one. Sutton put back Ty Johnson's miss with 0.9 seconds to tie the game at 60 and send it to overtime.
The Wildcats never should have let it come to the last shot of regulation. They committed turnovers on three straight possessions in the final minutes and Notre Dame pounced.
Villanova's ninth turnover led to an Irish bucket for a 56-51 lead. Villanova coach Jay Wright called a timeout on the next possession but the Wildcats had a shot-clock violation.
Connaughton buried a 3 to make it 56-54. Again the Wildcats turned the ball over, this time on a travel.
Atkins gave them a 57-56 lead on a three-point play before Grant hit a 3 for a sudden four-point lead, and the Notre Dame faithful, silenced all night, burst into a "Let's go Irish!" chant.
The Wildcats went more than 7 minutes without a basket until Sutton tied it.
Connaughton and Pinkston exchanged 3s in OT to make it 65-65, and the Irish still had big buckets left in them.
The Wildcats could have used this kind of first-half play starting two months ago. They scored the first 13 points of a 28-5 run that put them up 20.
Pinkston and Cheek hit consecutive 3s during that run and the Wildcats shot 50 percent from the floor for most of the half. Led by Mouphtaou Yarou, Villanova punished them on the boards, too, outrebounding the Irish 22-8. Yarou finished with 11 rebounds.
Villanova led 39-23 at halftime.