TVLine reports that the actress will play Jasmine, a "pushy and sexy" friend of Fiona (Emmy Rossum).
The character is currently expected to appear in the final two episodes of the show's first season.
Smart previously starred in WB series Felicity from 1999 to 2001 and also recurred in four episodes of Scrubs.
Her film credits include roles in the Crank films opposite Jason Statham and 2008 horror Mirrors, starring Kiefer Sutherland.
It was recently revealed that Sid and Nancy star Chloe Webb will appear in Shameless as the estranged ex-wife of Frank (William H. Macy).
The series continues on Sundays at 10pm on Showtime.