Williams' agent, Wynn Silberman, said Williams would fly to Kansas City later Wednesday to join his new team and be reunited with former San Francisco teammate quarterback Alex Smith.
The Chiefs (9-0) said they released wideout Chad Hall to clear roster room for Williams' arrival. Williams had already changed his Twitter handle to refer to himself as a "KC Chiefs WR" and posted a tweet that read only "Chiefs!"
Williams missed last season's run to the Super Bowl a 34-31 loss to the Ravens because of a serious knee injury that required surgery. He remained a key member of the team and was honored with the team's Ed Block Courage Award.