Self-decorated Mn 2 O 3 nanocrystals on Geobacter sulfurreducens were synthesized as sustainable and efficient water oxidation catalysts. The development of a low cost and efficient oxygen evolution reaction (OER) catalyst has paramount importance to meet the future sustainable energy demand. Nature's photosynthetic machinery deploy manganese-based complex in the photosystem II to oxidize water. Inspired by nature, herein, we synthesized a high performing manganese-based OER catalyst using an electrochemically active and iron-rich bacterium, Geobacter sulfurreducens . The as-synthesized biohybrid catalyst (amorphous Geobacter -Mn 2 O 3 ) produced a current density of 10 mA cm −2 at an overpotential of 290 ± 9 mV versus a reversible hydrogen electrode with a low Tafel slope of 59 mV dec −1 . The catalyst exhibited remarkable stability, evidenced through a long-term chronopotentiometry experiment. Multiple evidence showed that G. sulfurreducens contributed OER active elements (iron and phosphorus) to the biohybrid catalyst, and the as-synthesized Geobacter -Mn 2 O 3 is amorphous. The amorphous structure of the biohybrid catalyst provided a large electrochemically active surface area and excess catalytic sites for the OER catalysis. In addition, Mn 3+ present in the biohybrid catalyst is believed to be the precursor for oxygen evolution. The OER activity of the biohybrid catalyst outperformed commercial-Mn 2 O 3 , commercial-IrO 2 and most of the benchmark precious OER catalysts, thus supporting its suitability for large-scale applications. The proposed green approach to synthesize a biohybrid catalyst paves a new avenue to develop robust and cost-effective electrocatalysts for energy-related applications.