The 59-year-old Oscar-winning actor has been voicing the character since 1995 and told the secret on Graham Norton's talk show, reported The Independent.
Norton brought out a Woody doll and asked Hanks if its voice is his, to which the actor responded saying, "No, it's my brother Jim. There are so many computer games and video things and Jim just works on those all year long."
The "Forrest Gump" actor said whenever he had more important assignment than "Toy Story" he asked makers to get his brother Jim.
The film is scheduled for July, 2018 release.