The forthcoming comedy-drama stars Michael Ealy and Warren Kole as two bickering detectives who enter couples counselling to save their partnership.
Prison Break star Purcell will appear in the first season finale, according to The Huffington Post.
He will play John Crowell, a cocky detective who clashes with Travis (Ealy) and Wes (Kole). Purcell's recent credits include a 2011 episode of Castle and Jason Statham film thriller Killer Elite.
Ed Begley Jr has also been cast in Common Law - the St. Elsewhere star will appear in two episodes as bumbling therapist Dr Van Waals.
Sonya Walger (Lost) and Nora Zehetner (Heroes) will also appear on the show.
Common Law premieres on May 11 at 10pm on USA.
> Common Law premiere delayed by USA
Watch a promo for Common Law below: