The crystal structure of niraparib tosylate monohydrate Form I has been solved and refined using synchrotron X-ray powder diffraction data and optimized using density functional theory techniques. Niraparib tosylate monohydrate Form I crystallizes in space group P-1 (#2) with a = 7.22060(7), b = 12.76475(20), c = 13.37488(16) Å, α = 88.7536(18), β = 88.0774(10), γ = 82.2609(6)°, V = 1,220.650(16) Å3, and Z = 2 at 298 K. The crystal structure consists of alternating double layers of cations and anions (including the water molecules) parallel to the ab-plane. Hydrogen bonds are prominent in the crystal structure. The water molecule acts as a donor to two different O atoms of the tosylate anion and as an acceptor from one of the H of the protonated piperidine ring. The other piperidyl N–H acts as a donor to the carbonyl group of another cation. Surprisingly, there are no cation–anion N–H···O hydrogen bonds. The amide group forms as a N–H···O hydrogen bond to the anion and an intramolecular N–H···N hydrogen bond to the indazole ring. The powder pattern has been submitted to the International Centre for Diffraction Data for inclusion in the Powder Diffraction File™.