Control of magnetism has attracted interest in achieving low-power and high-speed applications such as magnetic data storage and spintronic devices. Two-dimensional magnets allow for control of magnetic properties using the electric field, electrostatic doping and strain. In two-dimensional atomically thin magnets, a non-volatile all-optical method would offer the distinct advantage of switching magnetic states without application of an external field. Here, we demonstrate such all-optical magnetization switching in the atomically thin ferromagnetic semiconductor, CrI₃, triggered by circularly polarized light pulses. The magnetization switching behaviour strongly depends on the exciting photon energy and polarization, in correspondence with excitonic transitions in CrI₃, indicating that the switching process is related to spin angular momentum transfer from photoexcited carriers to local magnetic moments. Such an all-optical magnetization switching should allow for further exploration of magneto-optical interactions and open up applications in high-speed and low-power spintronic devices.