Washington left the show in 2007 after an on-set clash, where he directed an anti-gay epithet to his fellow cast member T.R. Knight, who played Dr. George O'Malley.
The return coincides with the departure of series star Sandra Oh and her character, Dr. Cristina Yang, who was engaged to Washington's character, Dr. Preston Burke, during the series.
ABC has not announced when Washington's episode will be running.
During the controversy, Washington also used the slur backstage at the 2007 Golden Globe awards. He publicly apologized and tried to make amends by meeting with gay-rights organizations.
The Associated Press contributed to this report