Electron transport through semiconductor and metallic nanoscale structures, molecular monolayers, and single molecules connected to external electrodes display rectification, switch, and staircase functionality of potential importance in future miniaturization of electronic devices. Common to most reported systems is, however, ultrahigh vacuum and/or cryogenic working conditions. Here we introduce a single-molecule device concept based on a class of robust redox active transition metal (Os(II)/(III)) complexes inserted between the working electrode and tip in an electrochemical scanning tunneling microscope (in situ STM). This configuration resembles a single-molecule transistor, where the reference electrode corresponds to the gate electrode. It operates at room temperature in a condensed matter (here aqueous) environment. Amplification on-off ratios up to 50 are found when the redox level is brought into the energy window between the Fermi levels of the electrodes by the overpotential ("gate voltage"). The current-voltage characteristics for two Os(II)/(III) complexes have been characterized systematically and supported by theoretical frames based on molecular charge transport theory.