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:
Felicity: What's going on? Ray? Ray: Oliver Queen is the Arrow. Felicity: What? Ray: I have a 140 IQ and three PhDs, but it's pretty hard to insult my intelligence, but I'm pretty sure you just did. It all makes sense now. Your, ah, sudden disappearances, your apprehension of helping me with my mission. You've been working with the Arrow this whole time.