In 2001, Williams was traded to the St. Louis Rams on draft day in exchange for picks in the second and fourth rounds. Due to roster concerns, Williams switched to free safety. As one of the leaders of a much-improved defense, Williams got a chance to play in the postseason for only the second time in his career. In the Rams divisional playoff game against the Green Bay Packers prior to the Super Bowl, He returned two interceptions from Packers quarterback Brett Favre for touchdowns and recovered a fumble. Then in the NFC title game, he intercepted a pass from Philadelphia Eagles quarterback Donovan McNabb, with 2 minutes left in regulation, clinching the game and ensuring the Rams' berth in Super Bowl XXXVI. However, the Rams lost that game to the New England Patriots.