Terms were not disclosed, but the Kansas City Star had reported it be a three- year contract worth $9 million.
"We are excited about the opportunity to make Kevin a member of the Kansas City Chiefs," said head coach Romeo Crennel. "Kevin has been a part of successful teams in the past and knows what it takes to win. I am eager to add him to our offense."
Boss spent the 2011 season with AFC West-rival Oakland, catching 28 passes for 368 yards and three touchdowns in 14 games, 11 of which were starts.
In 72 games, including 56 starts, over five seasons with the Raiders and New York Giants, the 28-year-old Western Oregon product has 147 receptions for 1,968 yards and 21 touchdowns.