"I'll be a Husky for life," 55-year-old Petersen said Monday, "but now is the right time for me to step away from my head coaching duties, and recharge."

Petersen has been widely regarded as a top 10 coach in the NCAA since he took over at UDub in 2014 ... and NOBODY expected him to leave the program this soon.

But, Petersen -- who's piled up a 54-26 record the past 6 years at Washington -- says he's ready to transition into an advisory role with the school.

Washington officials announced Petersen's defensive coordinator, Jimmy Lake, will take over the head coaching duties after this season concludes.

"It has been a privilege and a professional dream fulfilled to be part of this world-class institution," Petersen said. "I will forever be grateful, honored and humbled to have had the opportunity to coach our fine young men on Montlake for these past six seasons."

Petersen will finish out the year as Washington's coach ... and is slated to step aside after the team's soon-to-be-announced bowl game.

Of course, Petersen gained fame for turning Boise State into a powerhouse ... going 92-12 from 2006-2013.