Earlier this week, Rams boss Wayne Rooney revealed that the club's administrators had agreed a fee for the prospect with a Premier League club.
While neither team have officially confirmed the deal, the 18-year-old has taken to social media to reveal that a transfer has been finalised.
Although the left-sided player has spent much of the campaign in the Under-23s, he has still been provided with four starts and four substitute outings in the first team this season.
Most notably, Williams played the full 90 minutes as Derby earned a goalless draw at West Bromwich Albion in September, as well as in the 1-0 win over Blackpool on December 11.
Williams, who has played at left-back, left wing-back and left midfield for Derby, has allegedly signed a six-year contract. body check tags ::