The empirical relationship between gross primary production (GPP) and evapotranspiration (ET) is well established at the local or point scale in the field of ecology. However, two questions remain unanswered: 1) does this relationship hold at the catchment scale? 2) what controls the characteristics of this relationship, such as the slope and intercept? We examine the monthly GPP, ET, precipitation, and streamflow data from observations or reanalysis for over 350 natural catchments across the contiguous United States and under various climate and landscape conditions. Further, we develop and solve a system of linear equations using circular statistics of hydroclimatic variables to explore the dominant factors driving seasonal GPP dynamics. We found that there is indeed a linear relationship between monthly GPP and ET at the catchment scale, with Pearson's r ≥ 0.5 and ≥ 0.9 for 98.5% and 72% of the catchments, respectively. We propose a two-parameter universal GPP-ET functional relationship, where the two parameters are estimated a priori as functions of catchment-scale climate and landscape conditions. Validation of the universal relationship showed that it performed reasonably well, with Kling-Gupta Efficient (KGE) ≥0.5 and ≥ 0.9 for ~98% and 51% of the catchments, respectively. More interestingly, the relationship showed systematic shifts with an increase in GPP and precipitation seasonality's, which can be further related to other climate and landscape conditions. Catchment wetness exerts primary controls on GPP seasonality in drier regions. In contrast, vapor pressure deficit and daily minimum temperature play the primary role in wetter regions. This functional relationship directly links catchment water balance to vegetation productivity; hence it can be used as a diagnostic tool for coupled water-carbon simulations at the catchment, regional or larger scales.