Knight is set for one episode in which he will play a suspected serial rapist who consistently maintains his innocence despite overwhelming evidence.
The drama series, according to NBC, "chronicles the life and crimes of the Special Victims Unit of the New York Police Department, the elite squad of detectives who investigate sexually based crimes."
Knight has been seen on Broadway in Noises Off, Tartuffe and A Life in the Theatre. He is best known for his work on "Grey's Anatomy," playing the role of Dr. George O'Malley, for which he earned an Emmy nomination in 2007.
"Law & Order: SVU" will return to NBC Sept. 21 and air Wednesday nights at 10 PM ET.