Therrien's dismissal came a day after the Penguins lost 6-2 at Toronto, setting their season record at 27-25-5, good for fourth place in the Atlantic Division. If the playoffs were to start now, Pittsburgh would be out of contention.
Therrien coached the team since December of 2005. The Penguins reached the Stanley Cup finals last season, losing to the Detroit Red Wings.
Bylsma, 38, a native of Grand Haven, Mich., moves up from the American Hockey League where he coached the Penguins' Wilkes-Barre-Scranton affiliate. Bylsma was an assistant coach with the NHL's New Islanders in 2005-06.
"We believe we need a change in direction and, with 25 games remaining in the regular season, our goal remains to finish strong and qualify for the Stanley Cup playoffs," Penguins General Manager Ray Shero said. "Dan Bylsma is one of the bright young coaches in the game and has done an exceptional job as the head coach in Wilkes-Barre-Scranton this season."