The Dallas native left the Longhorns just before the season started in November. He played in seven of Texas' first eight games as a freshman in 2009 before injuring an ankle.
Williams enrolled at SMU on Tuesday but will have to sit out until the end of the fall semester in December. He will have three years of eligibility remaining.
The 6-foot-7 forward was a high school standout in the Dallas area at Duncanville, which went 39-0 and won the Class 5A state title his sophomore year.