Vasher, who has played his entire six-year career with the Chicago Bears, will sign for $4.5 million over two seasons, a league source told ESPN NFL Insider Adam Schefter.
"In Nathan Vasher, we're adding a proven NFL player with experience in the league," said general manager A.J. Smith. "Having played in Chicago under Chargers defensive coordinator Ron Rivera [2004-06] and secondary coach Steve Wilks [2006-08] is an added bonus."
Vasher played in 15 games last season for the Bears, with one interception. He has 19 career picks in 73 career games. He made the NFC Pro Bowl team in 2005, when he had eight interceptions.
Vasher, 28, played collegiately at Texas, where he was a teammate of Quentin Jammer, who also plays cornerback for the Chargers.
Information from ESPN NFL Insider Adam Schefter is included in this report.