Knight, 38, will play a suspected serial rapist who consistently maintains his innocence despite overwhelming evidence.
Knight is best known for his portrayal of Dr. George O'Malley on Grey's Anatomy, for which he earned an Emmy nomination in 2007. During his five-year tenure, Knight was involved in an on-set argument when co-star Isaiah Washington made comments insulting Knight's sexuality in 2006. Knight subsequently came out, and Washington exited the series in 2007. Knight followed in 2009, when he asked to leave the hit medical drama and his character was killed off.
Since his time at Seattle Grace, Knight has starred in the Los Angeles production of the musical Parade and appeared on Broadway in the 2010 revival of David Mamet's A Life in the Theatre.
Knight's episode is in production. No airdate has been set.
The new season of Law & Order: SVU premieres Wednesday, Sept. 21 at 10/9c on NBC.