"Sixteen years after Brett Favre came to the Packers, he is returning for a seventeenth season," Packers' Chief Executive Officer and President Mark Murphy said.
"He has had a great career with our organization and although we built this year around the assumption that Brett meant what he said about retiring, Brett is coming back. We will welcome him back and turn this situation to our advantage."
Murphy said Favre would be in training camp Monday.
"Frankly, Brett's change of mind put us in a very difficult spot," Murphy said. "We now will revise many actions and assumptions about our long-term future, all predicated on Brett's decision last March to retire."
The team hasn't said who will be cut to make room for Favre, the Milwaukee Journal Sentinel said.
The Packers, who are to pay Favre $12 million for the season, have been adamant that he would be used in a backup role to Aaron Rodgers.
There have been reports the team had offered Favre more to stay retired and assume a non-player role with the franchise.
Favre, 38, must take a physical and a conditioning test before being put on the roster.