Polaritons are hybrid quasi-particles made of photons and excitons. They represent the fundamental electronic excitations of an insulator interacting with the electromagnetic field. In planar semiconductor microcavities, polaritons are confined along one spatial direction and can be described as two-dimensional quasi-particles possessing a very light effective mass of the order of 10-5 times the free electron mass. Their spatial confinement in the two lateral directions is a challenging task because of the large group velocity. On the other hand, spatial trapping would be beneficial for the long sought phenomenon of polariton Bose-Einstein Condensation, as it proved to be for BEC of alkali atoms. Here, we report on the first realization of a spatial trap for polaritons through local variations of a microcavity thickness, within micrometer sized circular regions. Due to their very light mass, confined polaritons display quantized energy levels that are delocalized in reciprocal space, already when trapped within a spatial region of several microns. We characterize these levels using angle-resolved emission spectroscopy and in comparison with a theoretical model. This novel structure proves very effective in producing spatial confinement and opens the way for the realization of mesoscopic devices showing quantum collective phenomena.