UltraDark.jl simulates cosmological scalar fields. Written in Julia, it is inspired by PyUltraLight (ascl:1810.009) and designed to be simple to use and extend. It solves a non-interacting scalar field Gross-Pitaevskii equation coupled to Poisson's equation for gravitational potential. The scalar field describes scalar dark matter in models including ultralight dark matter, fuzzy dark matter, axion-like particles and the like. It also describes an inflaton field in the reheating epoch of the early universe.