Benanti will play the fiancee of Colin Sweeney ( Dylan Baker), an accused wife killer and frequent Lockhart/Gardner client.
Josh Charles will direct the episode, which is scheduled to air in April or May.
Benanti's character is "beautifully suited for Colin Sweeney," executive producer Michelle King told TVGuide.com.
Benanti received Tony Award nominations for her work in Swing!, Into the Woods and Women on the Verge of a Nervous Breakdown and won Best Featured Actress in a Musical Tony for her performance in Gypsy. Her stage work also includes Nine, The Wedding Singer and In the Next Room.
Her TV work includes "The Playboy Club," "Law & Order: SVU," "Royal Pains" and "Go On." She recently played Elsa Schrader on NBC's The Sound of Music Live!