Allen has a history of concussions but it was not immediately known what prompted his departure from the sport at such a young age.
"I didn't see that coming," said Vikings coach Leslie Frazier. "Everybody has to make decisions that they think are best for them.
"His experience would have helped us, but he has made a decision that he thinks is best for he and his family and you have to respect that."
Allen, who was a third round selection in the 2009 draft, started in nine of his 12 appearances last season making 49 tackles and one interception.