In this study, highly dispersed Ag nanoparticles anchored on Fe3O4 modified polydopamine sub-micrometer spheres (PSS) nanocomposites were fabricated by a facile two-step approach. The catalytic properties of the as-obtained PSS/Fe3O4/Ag ternary nanocomposites were evaluated with the reduction of 4-nitrophenol by NaBH4 as a model reaction. It is shown that the presence of Fe3O4 can significantly decrease the particle size of the in-situ formed Ag nanoparticles on PSS, and optimize the catalytic performance of PSS/Ag nanocomposites. Additionally, the as-synthesized PSS/Fe3O4/Ag ternary nanocomposites exhibit high catalytic stability for recycle. Magnetic study reveals that the ternary nanocomposites display room-temperature superparamagnetic behavior. It is believed that this simple and effective method can be extended for the preparation of other polydopamine-based multicomponent nanocomposites with intriguing property, which would hold great promise for a variety of applications.