Mitigation of urban carbon dioxide (CO2) emissions is crucial to combat climate change. Although urban forests are expected to sequester atmospheric carbon, few studies have evaluated net CO2 fluxes of extensive urban vegetation. In order to assess the mitigation potential of an urban forest, we measured CO2 fluxes with an eddy covariance tower in the Timiryazevsky urban forest located in northern Moscow during the two vegetation periods 2014 and 2017 and analyzed them regarding diurnal and seasonal patterns. Results of the carbon budget indicate that the forest area was on average a CO2 source in every month, although in early summer it was consistently a sink during daytime and also at daily scale on few occasions. The warmer and drier vegetation period of 2014 was in general a stronger source than the cooler and wetter vegetation period 2017. The results expand the sparse evidence of urban forests’ CO2 fluxes, though the impact of anthropogenic source contributions to the measured fluxes cannot be completely excluded. In order to validate our findings, independent verification of net CO2 flux components, enhanced footprint calculations and observations spanning over larger periods of time should be the next step. © 2021, The Author(s), under exclusive license to Springer Nature Switzerland AG.