A novel vanadium hydroxide-phosphate, NH4VPO4OH, was synthesized hydrothermally in V2O5−NH4H2PO4-citric acid system at 230 °C. It was characterized by XRD, TG-DSC, SEM-EDX, FTIR and NMR spectroscopy. NH4VPO4OH is isostructural with NH4GaPO4OH and features edge-sharing chains of VO6 octahedra. These chains running along [010] direction of the unit cell are connected by phosphate tetrahedra to form infinite layers parallel to the (100) plane. Ammonium cations are embedded between the heteropolyhedral layers. According to the thermodynamic and NMR measurements supported by the first-principles calculations, NH4VPO4OH presents a rare case of Haldane spin system with spin S=1 based on V3+ ions. © 2023 Wiley-VCH GmbH.