The magnetic and optical properties of regioregular poly(3-hexylthiophene)-capped Au nanoparticles (NPs) have been investigated in order to clarify the effectiveness of polythiophene capping on the induction of ferromagnetism in Au. The room-temperature magnetization curve of the polythiophene-capped Au NPs exhibits a clear hysteresis behavior with a spontaneous magnetization of 8.5 x 10-3 emu/g. The average magnetic moment of the surface Au atoms is estimated to be 2.6 x 10-3 B. The ultraviolet-visible spectrum shows a clear sign of the surface plasmon absorbance of metallic Au which reflects the week character of the chemical bond between the Au and S atoms on capping surface. Our results show clearly that Au NPs capped with polythiophene can be ferromagnetic.