To investigate the nature of the 5f electrons in the noncollinear ferromagnet U3P4 the photon-frequency dependence of the normal-incidence reflectivity and the polar magneto-optical Kerr rotation are studied theoretically and compared with available experimental data. The calculations are carried out treating the U 5f electrons as itinerant and the results are found to be in good agreement with experimental data. We show that the U 5f states contribute substantially to the optical properties. We discuss the role played by the noncollinearity of the magnetic structure in determining the optical properties. We conclude that the magnetism of this compound can be described in terms of well-defined moments of the U atoms that are formed by itinerant U 5f electrons.