Gary Andersen informed the Wisconsin football squad at a team meeting at 4 p.m. on Wednesday that he was taking the head coaching job at Oregon State University.
Andersen becomes the second head coach to leave the program suddenly in two years, after Bret Bielema departed to take the head job at Arkansas in December 2012.
Wisconsin Director of Athletics Barry Alvarez immediately started a national search for a new head football coach.
"I began working to find a new head coach as soon as I spoke with Gary this morning," Alvarez said in a statement released by the university. "My first concern is taking care of the players on the current team, especially the senior class, and ensuring that their bowl experience is a memorable one. I will find a head coach to uphold the great tradition at Wisconsin, someone who is committed to excellence both on and off the field.
"I want to thank Gary for his two years here and commend him on the way his team performed on the field, in the classroom and in the community. I wish him the best at Oregon State."
The Badgers are fresh off a 59-0 loss to Ohio State in the Big Ten Championship game and have gone 19-7 under Andersen.
"First and foremost I want to thank Coach Alvarez for the opportunity to coach at the University of Wisconsin," Andersen said in a statement released by Wisconsin. "I also want to thank my staff and the people at UW. We worked very hard together and accomplished some great things. I had the opportunity to meet and coach some great young men and I look forward to watching them as they continue their careers and move through life."
In a press conference in Madison, Alvarez said "our program will not take a step backward."
Wisconsin is set to meet Auburn in the Outback Bowl on Jan. 1. Alvarez, who coached the team in the Rose Bowl after Bielema departed, has yet to decide if he will coach the team on New Years Day.