Polymer-inorganic composite particles composed of poly(diphenylamine) (PDPA) and Fe3O4 were synthesized to fabricate a smart material that can respond to both electric and magnetic fields. PDPA was synthesized using a chemical oxidative polymerization process, and the PDPA/magnetite (PDPA/Fe3O4) composites were fabricated by loading Fe3O4 nanoparticles onto the surface of PDPA using a coprecipitation method. The surface roughness of the composites was larger than that of pure PDPA, and the composites exhibited paramagnetic properties with a saturation magnetization of 77.1 kA/m. An electrorheological (ER) and magnetorheological (MR) fluid was prepared by dispersing PDPA/Fe3O4 composites in silicone oil. An examination of the responses under an external electric or magnetic field revealed significant ER and MR characteristics.