The Islanders made official Friday what had already become an open secret, hiring Benoit Desrosiers as their third assistant coach.

Desrosiers, who joins John MacLean and Doug Houda on Patrick Roy’s staff, worked with Roy for six seasons with the Remparts in the Quebec Maritimes Junior League, winning the 2023 Memorial Cup.

He became head coach of the Gatineau Olympiques before this season but resigned after 47 games to go to the Islanders.

This is the first job in the pros for the 35-year-old Desrosiers, who worked with the Sherbrooke Phoenix — also in the QMJHL — prior to joining Roy in Quebec.

He was initially reported to be joining the Isles within days after Roy’s hiring, but the team issued a statement shooting that report down, while saying Desrosiers would meet with Lou Lamoriello in the future.

Desrosiers is said to operate similarly to Roy, with Gatineau general manager Serge Beausoleil calling them “twins” in a conversation with The Post this week.

“He’s a smart guy,” Olympiques winger Justin Dumais told The Post. “He’s got Patty Roy with him. It’s nothing new that he hasn’t done before.”