On local scale, the eddy covariance technique is suited to estimate gross primary production (GPP). Scaling up such observations to the regional and continental level, however, remains a challenge. Here, we show that there is a surprisingly robust stoichiometric relationship between vegetation CO2 and H2O fluxes, mediated by vapor pressure deficit (VPD), across many different forest vegetation types. This relationship is used to provide a data-driven estimate of Europe's GPP from its water balance. Namely, watershed-wide evapotranspiration (ET), as derived from precipitation (P) and river runoff (R), is multiplied by the ratio of GPP to ET as derived from eddy covariance measurements (water-use efficiency, WUE). In doing so, GPP of Europe is estimated to range between 3.9 and 5.8 PgC/a (median 5 PgC/a). Such GPP estimate is an important independent benchmark for large-scale ecosystem models and may be extended to global scale when relevant data becomes available.