The actor will appear in the 18th episode of the NBC procedural's current season, TV Line says.
His character, who has taught himself law, will play a convict who insists that he is innocent.
Diaz has previously appeared in shows such as Mercy, Weeds, No Ordinary Family and Love Bites.
He will appear in ABC's upcoming drama Scandal, which is being executive produced by Grey's Anatomy creator Shonda Rhimes.
Chloe Sevigny also recently signed up for a guest role in Law & Order: SVU, which airs on Wednesdays on NBC.