The elemental stoichiometry of particulate organic carbon (C), nitrogen (N), and phosphorus (P) connects the C fluxes of biological production to the availability of the limiting nutrients in the ocean. It also influences the marine food-web by modulating the feeding behavior of zooplankton and the decomposition of organic matter by bacteria and viruses. Despite its importance, there is a general paucity of information on how the global C:N:P ratio evolves seasonally and interannually, and large parts of the global ocean remain devoid of observational data. Here, we developed a new method that combines satellite ocean-color data with a cellular trait-based model to characterize the spatio-temporal variability of the phytoplankton stoichiometry in the surface mixed layer of the ocean. Here, we demonstrated this method specifically for the C:P ratio. The approach was applied to phytoplankton growth rates and chlorophyll-to-carbon ratios derived from MODIS-Aqua and to maps of temperature-dependent nutrient limitation in order to generate global and seasonal maps of upper-ocean phytoplankton C:P. Taking it a step further, we determined the C:P of the bulk particulate organic matter, using MODIS-Aqua estimates of particulate organic carbon and phytoplankton biomass. A reasonably good comparison of our results with available data, both horizontal distributions and time series, indicates the viability of our new method in accurately quantifying seasonally resolved global ocean bulk C:P. We anticipate that the new hyperspectral capabilities of the NASA’s PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) mission will facilitate the determination of phytoplankton stoichiometry for different size classes and can further enhance the predictability of marine ecosystem stoichiometry from space.