"I can die happy now. Studio Ghibli fan from early childhood. Can't wait for this to come out," Musgraves tweeted Tuesday.
The 3D, computer-animated film was directed by Goro Miyazaki, whose credits include From Up on Poppy Hill and Tales from Earthsea.
GKIDS said it will release the Japanese film in North America in early 2021.
The English-language voice cast will also include Pandora Colin, Alex Cartañá, Taylor Paige Henderson, J.B. Blanc, Logan Hannan, Summer Jenkins, Vivienne Rutherford, Tom Bromhead, Eva Kaminsky and Vanessa Marshall.
The film is based on the book by Diana Wynne Jones, author of Howl's Moving Castle.