In announcing his decision, Williams, 66, said, "It's the right time."
"My entire career has been an unbelievable blessing. I am fiercely proud of the program we have built here," he said in a statement. "I couldn't have asked any more from my players, my assistant coaches, the great Maryland fans and this great university. Together, we did something very special here."
Williams, a Maryland alumnus, took over the Maryland program before the 1989-90 season. During the next 22 years, he guided the Terrapins to a 460-248 record, including 192-154 in the ACC. Maryland claimed conference championships under Williams in 1995, 2002 and 2010.
The 2001-02 season was the most remarkable, Maryland was 32-4 overall, losing just once in ACC play. The Terrapins defeated Indiana 64-52 in winning the NCAA tournament.
Maryland was Williams' fourth head coaching position. He started as American University, coaching there for four seasons before heading to Boston College for four years. He then moved to Ohio State for three seasons before taking over at Maryland.
His overall NCAA coaching record is 667-376.