The effect of nonlinear carrier recombination on the photothermal modulated optical reflectance (PMOR) signal of a crystalline semiconductor has been theoretically modelled. The PMOR signal consists of both thermal and Drude (free-carrier) components, and nonlinear recombination influences it through its direct effect on the modulated carrier density, and its indirect effect on the modulated sample temperature. Quadratic recombination was shown to lead to a sublinear intensity dependence of the photocurrent amplitude at elevated optical excitation intensities; cubic recombination leads to a supralinear intensity dependence of the PMOR amplitude accompanied by a progressive phase lag dependence on pump laser-beam intensity. The effect of nonlinear carrier and heat-flux gradients was also considered and found to be negligible with PMOR detection.