In this work, bimetallic AuPt alloy nanoparticles were synthesized in situ in polyelectrolyte multilayers (PEMs) via an ion-exchange and coreduction process, in which the PEM support also served to suppress the AuPt phase separation, and thus enabled formation of AuPt alloy nanoparticles over a wide composition range. The PEM supported AuPt alloy nanoparticles exhibited higher catalytic activity than Au and Pt monometallic ones for the reduction of 4-nitrophenol by NaBH4, showing synergistic effects between Au and Pt. This work provides a facile approach to in situ synthesis of polymer supported bimetallic nanoparticles of tailored composition for optimum performance in catalysis and other applications.