Since the discovery of graphene(1), the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin-valley coupling(2), Ising superconductors(3-5) that can be tuned into a quantum metal(6), possible Mott insulators with tunable charge-density waves(7), and topological semimetals with edge transport(8,9). However, no two-dimensional crystal with intrinsic magnetism has yet been discovered(10-14); such a crystal would be useful in many technologies from sensing to data storage(15). Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin-Wagner theorem(16). Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals(17-19). Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect(20), whereas in trilayer CrI3 the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics(12), and van der Waals engineering to produce interface phenomena(15).