A compact magnetless isolator for optical communication systems based on a ring resonator with an outer layer made of silicon and an inner layer made of a magneto-optical material that does not require an external magnet to keep its magnetization saturated is suggested. Three-dimensional computational simulations of the device performed with the full-wave electromagnetic solver COMSOL Multiphysics show that the insertion loss and isolation levels are about −1.9 dB and −23 dB, respectively, thus confirming the feasibility of the isolator. An analytical model of the device based on the temporal coupled-mode theory method has been formulated and there is a good agreement between the analytical and simulation results. Since it does not require a magnetization scheme based on permanent magnets or electromagnets as conventional isolators do, the presented isolator is much more compact, with a footprint at least one order of magnitude smaller in comparison with other isolator designs known from the literature, and this feature makes the suggested device an ideal candidate for optical circuits with very high integration density.