While Streep will be reprising her role as Donna in the movie, James will play a younger Donna at certain parts. The sequel takes place 10 years after the original 2008 film.
James is currently starring in Baby Driver alongside Ansel Elgort, and she is known for playing the titular character in Cinderella.
She'll join the returning Mamma Mia cast, which includes Amanda Seyfried and Christine Baranski.
If you're excited to see the musical cast reunite though, you'll have to wait until next year. The film is slated for a July 20, 2018, release.