The photochemical core of every photosynthetic apparatus is the reaction center, a transmembrane enzyme that converts photons into charge-separated states across the biological membrane with an almost unitary quantum yield. A light-responsive organic transistor architecture, which converts light into electrical current by exploiting the efficiency of this biological machinery, is presented. Proper surface tailoring enables the integration of the bacterial reaction center as photoactive element in organic transistors, allowing the transduction of its photogenerated voltage into photomodulation of the output current up to two orders of magnitude. This device architecture, termed light-responsive electrolyte-gated organic transistor, is the prototype of a new generation of low-power hybrid bio-optoelectronic organic devices.