A theory is presented for how magnetic nanostructures can catalyze intersystem crossing in molecular radical pairs. Magnetic field gradients near physically realistic nanostructures are strong enough to induce a relative reorientation of two electronic spins in < 1 ns, overwhelming nuclear hyperfine coupling as a driver of intersystem crossing. Nanomagnetic control of intersystem crossing represents a form of heterogeneous catalysis that does not require molecular contact, but only short-range magnetic coupling.