Linear and nonlinear optical properties of a spontaneously grown non-centrosymmetric crystal of guanylurea(1+) hydrogen phosphite (GUHP) were investigated. Optically negative biaxial crystals of GUHP exhibit a birefringence higher than calcite. Highly anisotropic refractive indices of GUHP and its nonlinear optical coefficients were determined. Phase matching of GUHP was described by an experimentally determined Hobden diagram. High and phase-matchable optical nonlinearities of GUHP together with its wide transparency range and an excellent resistance against optical damage approve its potential for applications in nonlinear optics.