Alex Nedeljkovic allowed five goals on 17 shots before being pulled in the second period, and Tristan Jarry made five saves in relief for the Penguins (18-18-8), who have lost five of six (1-2-3).
"I think we just got outplayed tonight," Pittsburgh coach Mike Sullivan said. "We didn't play well enough."
Ottawa scored on its first two shots on goal.
Tyler Kleven put the Senators ahead 1-0 at 3:16 of the first period, a one-timer from the point off a pass from Giroux. It was upheld after Pittsburgh, which has allowed a goal on the first shot in 10 of 44 games this season, used a coach's challenge for goalie interference.
Pinto made it 2-0 with two seconds remaining on the ensuing power play at 5:14. He ended a seven-game goal drought from the right face-off circle on another one-timer set up by Giroux.
Pinto extended the lead to 3-0 at 19:04 with a short-handed goal, keeping the puck on a 2-on-1 for a wrist shot from the right circle.
"Came out strong, had a good first," Pinto said. "Kind of shut it down. Leevi made some big saves. Just a good overall team win. ... Try to get our swagger back. We had it a bit on the West Coast (road trip in December). We kind of lost it for a bit. Hopefully, this one builds our confidence."