In this paper, we present a new efficient composite electrocatalyst, manganese dioxide-supported silver bismuthate (Ag4Bi2O5/MnO2), for oxygen reduction reaction (ORR) in alkaline media. The new electrocatalyst was characterized with scanning electron microscope (SEM), powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Electrochemical measurements indicate that the Ag4Bi2O5/MnO2 composite is a very efficient electrocatalyst for ORR in alkaline media. The physical and electrochemical characterization results suggest that the high activity is ascribed to the support effects from MnO2 and the synergetic effects among Ag4Bi2O5 and MnO2. The analysis of rotating disk electrode (RDE) results shows that the ORR occurs via a four-electron pathway on the surface of the Ag4Bi2O5/MnO2 electrocatalyst. This electrocatalyst was further tested in a designed zinc–oxygen (Zn–O2) battery. This battery can offer a discharge time of 225h at 120mAcm−2, increasing by more than 492% as compared with pure MnO2 electrocatalyst. It demonstrates that this inexpensive Ag4Bi2O5/MnO2 electrocatalyst is a viable alternative to platinum electrocatalyst for energy conversion devices.