Williams will redshirt next season and have two years of eligibility remaining. He spent 1½ seasons with Tennessee before being granted his release from the school last month.
Rams coach Mike Bobo recruited Williams when Bobo was the offensive coordinator at Georgia. Williams, who is from Lovejoy, Georgia, was rated as the No. 38 prospect in the Class of 2015 by ESPN.
Williams caught nine passes for 89 yards this season. He had seven catches for 158 yards and two touchdowns as a freshman.
The Associated Press contributed to this report.