Where to find Zygarde in Sun, Moon, Ultra Sun & Ultra Moon

Gift: Route 16
Static Encounter: Resolution Cave (Only in Ultra Sun & Ultra Moon)

Zygarde also appears in

How to get Zygarde in Sun, Moon, Ultra Sun & Ultra Moon

Its base catch rate is 3 out of 255 (higher is easier); the catch rate calculator works out your odds with each ball.

Zygarde in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)