A combination of radiotracer and pore‐water concentration measurements provide evidence for the occurrence of acetogenesis from CO2 in anoxic marine sediments that are ordinarily dominated by sulfate reduction and methanogenesis. In a month‐long incubation experiment using sediments from Cape Lookout Bight, North Carolina, we measured H2 and acetate concentrations and monitored the incorporation of 14CO2 into 14CH4 and 14C‐acetate. Depletion of pore‐water sulfate resulted in a period of elevated H2 concentrations that made acetogenic CO2 reduction thermodynamically favorable. During this period, 14C‐acetate was produced from 14CO2 at rates comparable to those of methanogenesis or sulfate reduction during their respective periods of dominance in the incubation. Maintenance of elevated but constant H2 concentrations immediately following sulfate depletion likely reflects control by acetogenic bacteria, suggesting they were the dominant consumers of H2 during this period.