The 91-year-old played chimney sweep Bert in the 1964 movie.
He has now revealed he has joined the cast of the new film Mary Poppins Returns, which stars Emily Blunt in the lead role.
He told The Hollywood Reporter: "This one supposedly takes place 20 years later and the kids are all grown up.
"It's a great cast - Meryl Streep, Angela Lansbury and that guy from Hamilton [Lin-Manuel Miranda]."
Van Dyke, who appeared alongside Julie Andrews in the original film, also said he has no plans to retire from performing.
"I think it's the worst thing you can do," he said. "Certain people who do retire suddenly age. I think you have to stay active."
Mary Poppins Returns is due to be released on Christmas Day 2018.
It will tell the story of an older Jane and Michael Banks and Michael's three children, who are visited by Poppins in the aftermath of a "personal loss" in Depression-hit London.
The film's director is Rob Marshall, who was also behind the camera for Into The Woods and the 2002 Oscar-winning film adaption of Chicago.
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Dick Van Dyke, who starred in the original Mary Poppins film, has confirmed he will appear in the movie's upcoming sequel.