Although the nutrient content of planktonic organic matter (C:N:Porg) plays a crucial role in marine metazoan evolution and global biogeochemistry (1–3), its geologic history is poorly constrained, and it is often regarded as a constant “Redfield” ratio of C:N:Porg~106:16:1. We calculate C:N:Porg through the Phanerozoic by including nutrient- and temperature-dependent C:N:Porg parameterizations (4–6) in a model of long-term biogeochemical cycles (7). We infer a decrease from high Paleozoic C:Porg and N:Porg to present-day Redfield ratios. This gradual nutrient enrichment of marine organic matter stems from a decrease in the global average temperature and an increase in seawater phosphate availability, which are driven by various Phanerozoic events, mainly the middle to late Paleozoic emergence and expansion of land plants and the Triassic breakup of the supercontinent Pangaea. The nutrient enrichment of planktonic organic matter likely impacted the evolution of marine fauna and global biogeochemistry.