The electro-biocatalytic conversion of CO2 into formic acid using whole-cell and isolated biocatalysts is useful as an alternative route for CO2 sequestration. In this study, Shewanella oneidensis MR-1 (S. oneidensis MR-1), a facultative aerobic bacterium that has been extensively studied for its utility as biofuel cells as well as for the detoxification of heavy metal oxides (i.e., MnO2, uranium), has been applied for the first time as a whole-cell biocatalyst for formic acid synthesis from gaseous CO2 and electrons supplied from an electrode. S. oneidensis MR-1, when aerobically grown in Luria-Bertani (LB) medium, exhibited its ability as a whole-cell biocatalyst for the conversion of CO2 into formic acid with moderate productivity of 0.59 mM h(-1) for 24 h. In addition, an optimization of growth conditions of S. oneidensis MR-1 resulted in a remarkable increase in productivity. The CO2 reduction reaction catalyzed by S. oneidensis MR-1, when anaerobically grown in newly optimized LB medium supplemented with fumarate and nitrate, exhibited 3.2-fold higher productivity (1.9 mM h(-1) for 72 h) compared to that grown aerobically in only LB medium. Furthermore, the average conversion rate of formic acid synthesis catalyzed by S. oneidensis MR-1 when grown in the optimal medium over a period of 72 h was 3.8 mM h(-1) g(-1) wet-cell, which is 9.6-fold higher than that catalyzed by Methylobacterium extorquens AM1 whole-cells in our previous study.