Integrated switching devices comprise the building blocks of ultrafast optical signal processing(1,2). As the next stage following intensity switching(1,3,4), circular polarization switches(5-9) are attracting considerable interest because of their applications in spin-based architectures(10). They usually take advantage of nonlinear optical effects, and require high powers and external optical elements. Semiconductor microcavities provide a significant step forward due to their low-threshold, polarization-dependent, nonlinear emission(11,12), fast operation(13) and integrability. Here, we demonstrate a non-local, all-optical spin switch based on exciton-polaritons in a semiconductor microcavity. In the presence of a sub-threshold pump laser (dark regime), a tightly localized probe induces the switch-on of the entire pumped area. If the pump is circularly polarized, the switch is conditional on the polarization of the probe, but if it is linearly polarized, a circularly polarized probe fully determines the final polarization of the pumped area. These results set the basis for the development of spin-based logic devices, integrated in a chip(14).