- New Mutants (Only appearance)
- Demon Bear (Only appearance)
- Essex Corporation (Last appearance)
- Smiley Men (Only appearance)
- Reverend Craig Sinclair (Only appearance) (Illusion)
- Thomas Guthrie (Only appearance) (Illusion)
- Emmanuel Da Costa (Mentioned)
- X-Men (Mentioned)
- Professor X (Charles Xavier) (Mentioned)
- Juliana Sandoval (Only appearance) (Illusion)
- Lockheed (Only appearance)
Races and Species:
- Earth-TRN414 (Last appearance)
Plot not yet listed
- Maisie Williams as Rahne Sinclair
- Rahne's wolf form is portrayed by Chuck
- Anya Taylor-Joy as Illyana Rasputin
- Colbi Gannett as Young Illyana Rasputin
- Charlie Heaton as Sam Guthrie
- Alice Braga as Dr. Cecilia Reyes
- Blu Hunt as Dani Moonstar
- Henry Zaga as Roberto Da Costa
- Adam Beach as William Lonestar
Additionally, Thomas Kee portrays Sam's father, Thomas. Happy Anderson appears as Reverend Craig Sinclair. Dustin Ceithamer physically portrays the Smiling Men, while singer Marilyn Manson provides their voice. Jacinto "Spirit Wolf" Vega appears as a frozen Cheyenne.
- This film has had multiple delays for its US release date:
- 20th Century Fox originally announced the April 13, 2018 release date for this film in April 2017, alongside dates for the films Deadpool 2 and Dark Phoenix.
- On January 11, 2018, the release date was pushed back to February 22, 2019.
- On March 26, 2018, the release date was pushed back to August 2, 2019.
- On May 7, 2019, the release date was pushed back to April 3, 2020.
- On March 12, 2020, the release date was pushed back due to the COVID-19 pandemic. On May 13, 2020, a new and final release date of August 28, 2020 was announced.
- On September 16, 2017, director Josh Boone announced that the principal photography had wrapped for this film.
- When Dani has a vision of an Essex Corporation facility where young mutants are being tested and experimented on, the footage used is the Transigen Project footage that Logan discovers in the film Logan. This could possibly hint that the two films take place within the same continuity. However, in Logan, Transigen is depicted to be under the Alkali Corporation and Dr. Zander Rice, and no mention is made regarding the Essex Corporation.
- Rosario Dawson, who portrayed Claire Temple in the Marvel Cinematic Universe, was originally reported to be in talks for the role of Dr. Cecilia Reyes.
- Jon Hamm was originally planned to make a brief appearance in the film as Mister Sinister, but said plans were scrapped in favor of introducing the character in the now canceled film Gambit, with the role and scene subsequently replaced by Antonio Banderas portraying a different villain.
- Warlock was featured in early drafts of the film's script, but was scrapped due to being deemed too expensive for the film by Fox.
- This film marks the last one in the X-Men film property made by 20th Century Fox, with the property returning to Marvel Studios following Disney's acquisition of Fox in March 2019.
- According to the co-writer and director Josh Boone, this film was planned to be the first in a trilogy of films. The second film would've introduced Karma, Warlock, and Antonio Banderas portraying Emmanuel Da Costa as a member of the Hellfire Club. While the third film would've introduced Darkchilde, and been a crossover with the main X-Men films, adapting the Inferno storyline. According to Boone, after the Disney acquisition, any scenes that were setting it up got cut during post-production.