Industrial production of phototrophic microorganisms is often hindered by low productivity due to limited light availability and therefore requires large land areas. This letter demonstrates that supply of hydrogen gas (H2) increases in phototrophic biomass productivity compared to a culture growing on light only. Experiments were performed growing Synechocystis sp. in batch bottles, with and without H2 in the headspace, which were exposed to light intensities of 70 and 100 μmol/m2/s. At 70 μmol/m2/s with H2, the average increase in biomass was 96 mg DW/L/d, whereas at 100 μmol/m2/s without H2, the average increase in biomass was 27 mg DW/L/d. Even at lower light intensity, the addition of H2 tripled the biomass yield compared to growth under light only. Photoreduction and photosynthesis occurred simultaneously, as both H2 consumption and O2 production were measured during biomass growth. Photoreduction used 1.85 mmol of H2 to produce 1.0 mmol of biomass, while photosynthesis produced 1.95 mmol of biomass. After transferring the culture to the dark, growth ceased, also in the presence of H2, showing that both light and H2 were needed for growth. A renewable H2 supply for higher biomass productivity is attractive since the combined efficiency of photovoltaics and electrolysis exceeds the photosynthetic efficiency.