Magnetically recyclable Au@Co core-shell nanoparticles were successfully synthesized in a one-step seeding-growth process within a few minutes. They were thermally stable and exhibited higher catalytic activity toward the dehydrogenation of ammonia borane than Au-Co alloy and the pure metal counterparts. This is a large enhancement in the catalytic activity of core-shell structured nanoparticles and will provide a new design principle for heterogeneous catalysis.