Brexpiprazole (BPZ) is an atypical antipsychotic drug indicated for the treatment of schizophrenia and depression. Crystal form screening of BPZ resulted in the formation of three polymorphs (I, II, and III), two methanol solvates, a toluene hemisolvate, and a dihydrate. Thermal analysis and solvent-mediated conversion experiments of the three unsolvated polymorphs established that Form I is the thermodynamically stable form at ambient temperature. All three polymorphs are monotropically related, whereby Forms I and II are the most and least stable forms, respectively. Structural diversity of BPZ was compared with two chemically related analogues, aripiprazole (APZ) and its active metabolite dehydro-aripiprazole (dAPZ). Like APZ and dAPZ, BPZ was shown to form a thermodynamically stable, hydrated crystal form when exposed to an aqueous environment; however, while APZ and dAPZ were characterized as monohydrates, BPZ is a dihydrate. The crystal structure of BPZ dihydrate (S2H2O) showed two water molecules c...