Believing he's mentally ill, rather than gifted, Haller starts out at a psychiatric institution.
There's no word yet whether Clement will be a patient or doctor there, a fellow hero or even a villain, but it has been confirmed he'll be in multiple episodes, Deadline reports.
Clement's pal and What We Do In the Shadows co-star and co-director Taika Waititi is also a part of the Marvel universe, currently directing Thor: Ragnarok in Australia.
Clement is also lending his voice to Disney's first film featuring a Pacific princess, Moana, in which he plays Tamatoa, a self-absorbed monster crab.
Legion debuts on FX early next year.