Disclosed are crystalline forms of sepiapterin free base selected from polymorphs A, B, C, D, E, F, and G, and combinations thereof, as well as crystalline polymorphs of salts of sepiapterin. Also disclosed are pharmaceutical compositions containing one or more such polymorphs and methods for preparing such polymorphs. Sepiapterin is useful in the treatment of a number diseases associated with low cellular levels of BH4, for example, phenylketonuria.