The untitled project, which is being developed by ABC, focuses on professional fixer Olivia Price (Kerry Washington).
Deadline says that Diaz has now joined the cast as a paralegal. The character is also said to be an "expert hacker" who does what he's told.
Diaz has previously starred in shows including Weeds and Mercy and currently has a recurring role in No Ordinary Family.
The Rhimes pilot will also feature Lost alum Henry Ian Cusick as a litigator.