We present a new mechanism--discovered using molecular dynamics simulations--that leads to complete healing of nanocracks. This mechanism relies on the generation of crystal defects known as disclinations by migrating grain boundaries. Crack healing by disclinations does not require any compressive loads applied normal to the crack faces and even occurs under monotonic tensile loading. By closing small cracks and suppressing the propagation of others, this mechanism may provide a novel way of mitigating internal damage that influences ductility in nanocrystalline metals.