Magnetic Pd–Ni bimetals with various compositions were synthesized by a one-pot reaction, in which chain-like Pd–Ni bimetals formed through the self-assembly of Pd–Ni nanoparticles. The self-assembly behaviour of the Pd–Ni bimetals gradually changes with the changes in composition. The Pd–Ni nanoparticles form simple chains at lower Pd content, while the Pd–Ni bimetals take on a beads-on-string structure composed of sphere-shaped Pd–Ni-nanoparticle assemblies at higher Pd content. Magnetic hysteresis loops show that Ni-rich assemblies have a higher saturation magnetization than Pd-rich assemblies. The formation mechanism for these Pd–Ni assemblies is proposed, in which the self-assembly behaviours are controlled by the competition of the surfactant interaction and the magnetic dipole interaction. The catalytic activity and selectivity of the hydrogenation of acetophenone over these Pd–Ni bimetals depend on their compositions.