The planet Ahch-To in the Ahch-To system, which itself resided within the Unknown Regions of the galaxy, was orbited by a moon: the moon of Ahch-To.
The initial appearance of Ahch-To's moon was in the 2017 film Star Wars: Episode VIII The Last Jedi. Its identification occurred in Star Wars: The Last Jedi: A Junior Novel, a junior novelization of the movie penned by Michael Kogge and released in 2018.