Barrowman, 48, confirmed during an appearance at the Amazing Las Vegas Comic Con over the weekend that he has finalised his deal to return, reported Ace Showbiz.
"We start filming season 4 in July, I just finalised my deal about a week ago. It was just about working out dates and stuff," he said.
"I'm very excited about going back. I can't tell you anything, because I do know a bit of what's gonna happen, but if you love Malcolm Merlyn, if you love Oliver Queen, if you love Thea Queen, if you love the Canary sisters ... I say that with a bit of a twist and a joke, you're gonna love what's gonna happen across the DC world."