An unusual 4f -superantiferromagnetic state characterized by simultaneous antiferromagnetic and spin-glass behaviors induced by particle-size reduction is revealed in metallic nanoparticles (≈ 9 nm) of TbCu 2 . The Néel temperature is 46 K and the glassy freezing is below ≈ 9 K and governed by a critical slowing down process. Neutron diffraction at 1.8 K establishes the superantiferromagnetism. The latter is settled by the nanoparticle moments and the freezing mechanism is provided by the surface spins.