Tropical rainforests exhibit some of the highest net primary production (NPP) rates in the world. It is widely hypothesized that soil nutrient availability, particularly phosphorus (P), limits NPP and influences the allocation of productivity between above- and below-ground components in tropical forests. However, these hypotheses have rarely been tested experimentally in extremely wet tropical rainforests. Here, we evaluated the effects of soil fertilization on above- and below-ground NPP in two localities in the Chocó region (Colombia), among the rainiest forests in the world, using five fertilization treatments (N, P, K, NPK, and control) applied in permanent plots. Contrary to expectations, total NPP did not respond to the addition of N, P, or K, despite the very high rainfall and extremely nutrient-poor soils. While total NPP remained unchanged, N fertilization significantly increased canopy NPP, measured as litterfall biomass. In addition, P fertilization induced a redistribution of NPP between above- and below-ground compartments, increasing the relative contribution of fine roots and litterfall while decreasing allocation to woody biomass. These shifts in carbon allocation likely represent an ecological strategy to enhance nutrient acquisition in soils with extremely low phosphorus availability.