The 6-2, 242-pounder visited with the Lions on March 19, and was the third free agent linebacker the Lions brought in as a possible replacement for Chris Claiborne.
The four-year NFL veteran has been very active in his pro career. He averaged 156 tackles over that span, and started 58 of 62 contests for the Jaguars and Cleveland Browns.
The product of Virginia played his first three seasons with the Browns before he was sent to Jacksonville during a draft-day trade. He anchored the Jaguars defense with 188 tackles, 87 solo, one quarterback sack, and five passes defensed last season.
He originally was drafted by Cleveland in the fourth round in 1999, and was thrust into the starting lineup as a rookie when Chris Spielman was forced to retire from the Browns. Rainer stepped up and led Cleveland with 191 tackles.
Also, he was honored by Kaleidoscope Magazine as one of 40 top professionals who have made an impact on their community, and was named the Browns' 2001 Man of the Year for his work with charities.
Rainer becomes the third unrestricted free agent to join the Lions this off-season. Detroit also inked cornerback Dre Bly and running back Shawn Bryson.