Michael J Fox was joined by Lea Thompson, Christopher Lloyd and Thomas Wilson in Boston for a "special" moment.
Michael J Fox and his co-stars got together again, 33 years after making the first movie, at the Fan Expo Boston convention.
Lea Thompson, who played his mother, shared the picture, and said: "Four old friends get back from the future."
Lloyd, who starred as inventor Dr Emmett "Doc" Brown, also shared the photo, with the comment: "This was special."
Wilson, best known as being villain Biff, said: "Wow. This just happened."
The cast were part of a panel discussion as to whether the four of them would return for another Back To The Future film.
Wilson joked: "Basically, I think America is saying, 'Come on they've wrecked every other franchise with bad sequels, why not this one? C'mon, we would watch it until it sucks.'"
Fox, now 57, said Bob Gale, the franchise's co-creator, has become its "gatekeeper".
Gale and director Robert Zemeckis have previously said another film is unlikely.
In October 2015, Lloyd reprised his role for a special short film which marked the franchise's 30th anniversary, and also the date to which Marty McFly and his girlfriend Jennifer travel back to in the 1985 and 1989 films.