Steele will co-star opposite Christine Baranski (Diane) and Cush Jumbo in the spinoff, co-created and executive produced by "The Good Wife" creators Robert and Michelle King, reported Deadline.
The spinoff will pick up one year after the events of the final episode of the show, with Baranski and Quinn reprising their roles as Diane Lockhart and Lucca Quinn, respectively.
The new series will revolve around an enormous financial scam that has destroyed the reputation of a young lawyer, while also wiping out her mentor Diane Lockhart's savings.
Steele will reprise her role as Marissa Gold. In the spin-off, she is a brilliant but scattered millennial who begins as Diane Lockhart's secretary, but finds she has a knack for investigating.
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The character will continue the original series' tradition of strong and sassy female investigators, following in the footsteps of Kalinda and Robyn.
"We're absolutely thrilled that Sarah will be joining the show. She's a magnificent comedian with the dramatic skill to keep every scene real," Robert and Michelle King said.
The 10-episode spinoff will premiere in February.