We use computational and experimental methods to study ferromagnetism at a charge-imbalanced interface between two perovskites. In superlattice calculations, the charge imbalance introduces holes in the layer, inducing a ferromagnetic half-metallic 2D hole gas at the interface oxygen orbitals. The charge imbalance overrides doping by vacancies at realistic concentrations. Varying the constituent materials shows ferromagnetism to be a general property of hole-type <?format ?> perovskite interfaces. Atomically sharp epitaxial , , and interfaces are found to exhibit ferromagnetic hysteresis at room temperature. We suggest that the behavior is due to the high density of states and exchange coupling at the oxygen band in comparison with the more studied band symmetry electron gas.