Colton Haynes will return to Arrow in the role of Roy Harper for an arc in the currently-on-midseason-break season 6.
“We’re thrilled to announce that Colton Haynes will be reprising his role as Roy Harper for a short arc on Arrow this year,” said executive producers Marc Guggenheim and Wendy Mericle in a statement (via Deadline). “Roy is called back to Star City to help resolve an urgent matter with high stakes for the team. His short visit turns into something surprising when he reunites with Thea and sparks a change in both of their lives that will have long-lasting consequences…”
Hayes - and Harper - departed the show at the end of the third season. He last appeared in the season 4 episode "Unchained."
No information about in which episode Haynes will return, nor in how many episodes he'll appear was given. Arrow returns from its midseason break on January 18, 2018.