The former Medium star will appear on Arrow Season 3 as Mark Shaw, an A.R.G.U.S. operative who turns into the DC Comic character Manhunter.
According to The Hollywood Reporter, Shaw will cross paths with Diggle and have a connection to The Suicide Squad.
As teased at Comic-Con last month, DC villain Ra's al Ghul will be the Big Bad this fall on the hit CW drama, with executive producer Marc Guggenheim teasing that this version of the villain will differ from the one seen in Christopher Nolan's Batman films.
"We really wanted our Ra's to be very different - at least our own Arrow-ified version," Guggenheim said.
Other big additions to Arrow Season 3 (premiering October 8) include:
Ray: Advanced Technology Operating Mechanism. ATOM. Felicity: What are you going to use it for? Ray: To protect the people of this city. And I want you to help. Felicity: Why does this keep happening to me?