Abstract The current study reports on biological biogas upgrading by means of hydrogen addition to obtain biomethane. A mesophilic (37 °C) 0.058 m3 trickle-bed reactor with an immobilized hydrogenotrophic enrichment culture was operated for a period of 8 months using a substrate mix of molecular hydrogen (H2) and biogas (36–42% CO2). Complete CO2 conversion (> 96%) was achieved up to a H2 loading rate of 6.5 mn3 H2/m3reactor vol. × d, corresponding to 2.3 h gas retention time. The optimum H2/CO2 ratio was determined to be between 3.67 and 4.15. CH4 concentrations above 96% were achieved with less than 0.1% residual H2. This gas quality complies even with tightest standards for grid injection without the need for additional CO2 removal. If less rigid standards must be fulfilled H2 loading rates can be almost doubled (10.95 versus 6.5 mn3 H2/m3reactor vol. × d) making the process even more attractive. At this H2 loading the achieved methane productivity was 2.52 mn3 CH4/m3reactor vol. × d. In terms of biogas this corresponds to an upgrading capacity of 6.9 mn3 biogas/m3reactor vol. × d. The conducted experiments demonstrate that biological methanation in an external reactor is well feasible for biogas upgrading under the prerequisite that an adequate H2 source is available.