The 20-year-old Williams is apparently the first player to capitalize on a court decision that made Clarett eligible for the draft.
The NFL requires a player to be out of high school three years before entering the draft. Clarett, a running back, challenged the requirement and U.S. District Court Judge Shira Scheindlin recently ruled in his favor. The league has appealed the decision.
"I believe this is the right time for me to go to the NFL," Williams said. "Nothing is guaranteed in life and opportunities like this don't always come. I see this as an opportunity and I want to make the most of it."
Williams, who caught 176 passes for 2,570 yards and 30 touchdowns in his two seasons with the Trojans, is expected to be among the top picks in the April 24-25 draft.
Under the NFL rule, Williams would have had to wait until 2005 to enter the draft.