Coach Mike Sullivan confirmed the positive tests, and said Pettersson is symptomatic while Ruhwedel is so far asymptomatic.
Pettersson and Ruhwedel are the fifth and sixth members of the Penguins to enter the protocol so far this season, joining forwards Jeff Carter, Jake Guentzel and Zach Aston-Reese and defenseman Kris Letang.
Letang, who was symptomatic, has been cleared to return to the team though it's still too early to say whether he'll be available Thursday when the Penguins host Philadelphia. Carter, Guentzel and Aston-Reese have returned.
"I don't think it's because guys aren't diligent, or aren't doing the right thing," Sullivan said. "I just think this is part of the world we're living in right now. It's a little bit different this year, because there's some semblance of normalcy.
"The protocols aren't mandated. But I think our guys do a really good job in just using common sense."
On Saturday, the Penguins saw the return of captain Sidney Crosby, who missed the start of the season recovering from wrist surgery. They are still without forward Evgeni Malkin, who is out until at least Thanksgiving while recovering from June knee surgery.
The Associated Press contributed to this report.