Details about the "Harry Potter" star's role in the movie version of the hit Netflix series are not yet known, but the actor will join cast members Ellie Kemper, Jane Krakowski, Tituss Burgess, and his "A Young Doctor's Notebook" co-star Jon Hamm for the project.
Set to hit the streaming service in 2020, the new film will give audiences the chance to choose different storylines.
Series creators Tina Fey and Robert Carlock are also working on the venture.
"We're thrilled to announce that we're about to start production on an all-new interactive 'Unbreakable Kimmy Schmidt Special', set to debut on Netflix in 2020," Fey said in a statement last month.
"'Unbreakable Kimmy Schmidt' was one of the first original comedy series on Netflix, and now it will be its first interactive comedy event. Fans will be able to make choices on behalf of our characters, taking different story paths with, of course, different jokes. I think it's a great fit for our show and will be a great way to officially complete the series".
The fourth and final season of "Unbreakable Kimmy Schmidt" was released earlier this year.
Radcliffe's addition to the cast of the upcoming special will allow the 29 year old to hone his comedy skills, after mostly veering towards dramatic roles or darker projects like "Victor Frankenstein" and "Imperium" since the "Harry Potter" franchise ended.
However, he did recently get the chance to show off his funny side in the anthology series "Miracle Workers", and has previously appeared in Amy Schumer's 2015 hit "Trainwreck", and 2013 romantic comedy "What If", as well as lending his voice to guest roles in animated series "The Simpsons", "Robot Chicken", and "BoJack Horseman".