The third-round 2009 draft pick out of Georgia didn't give a reason for his decision, but he has been plagued by a history of concussions.
"I didn't see that coming," Vikings Coach Leslie Frazier said. "Everybody has to make decisions that they think are best for them."
Frazier said the Vikings could have benefited from Allen's experience, "but he's made a decision that he thinks is best for he and his family and you have to respect that."
The cornerback started nine games last season, playing in 12 overall, during which he made 49 tackles and logged an interception.
In three years with Minnesota, Allen racked up 134 tackles and four pick-offs.