ESPN reports:

Detroit Lions star receiver Calvin Johnson told his family and a close circle of friends before the past season that 2015-16 would be his final season in the NFL. He delivered the same message to coach Jim Caldwell the day after the regular season ended, sources told ESPN.
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Johnson’s body has been so sore and his conviction so strong that he shared his decision to retire after the 2015 season with only two teammates — quarterback Matthew Stafford and linebacker Stephen Tulloch — with the request that they keep it confidential, according to sources.
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[U]nless Johnson has the change of heart the Lions are still hoping for (but not many are expecting), one of the greatest players in Detroit franchise history is likely to walk away from the game.

The full report is available here.

As difficult as it is to imagine these Lions without their all-pro receiver, a change of heart on the retirement decision seems like a long shot, even by Johnson’s standards. While the team hasn’t given Lions fans much to cheer about in terms of postseason success and championship contention, like Barry Sanders before him, Johnson treated us to the special experience of cheering for one of the game’s all-time greats clad in Detroit’s Honolulu blue and silver.

At the time of this post, Johnson has not made a definitive public statement on the subject of his retirement, and neither have the Lions.