Tropical forests are thought to have among the highest values of net primary productivity (NPP) on Earth, but comprehensive data on NPP have rarely been collected for tropical forests, especially outside of the Neotropics. In this study, we quantify aboveground and belowground NPP, along with additional environmental factors over a 1–2 year period in a lowland dipterocarp forest in Borneo. The site is characterized by high annual rainfall and typically no month of the year when the forest is under water stress. We estimated the total NPP to be 15.89 ± 0.90 Mg C ha−1 yr−1 (mean ± standard error) for a forest plot on clay soils and 12.79 ± 0.68 Mg C ha−1 yr−1 for a forest plot on sandy loam soils. Of this productivity, the allocation to aboveground NPP is 81.8 ± 6.0%/80.4 ± 6.0% and to belowground NPP is 18.2 ± 3.7%/19.6 ± 2.6% on clay and sandy loam, respectively. Fine root productivity (NPPfine roots) shows stronger seasonal variation relative to other components of NPP. There is evidence suggesting interannual variation in NPPfine roots, leaf flush, NPPlitterfall, and NPPACW. This is the first attempt to describe how the biomass of a Bornean tropical forest is allocated to the various components of NPP over the seasonal cycle. The study highlights the marked seasonality of a tropical forest even under largely aseasonal environmental conditions, as well as the important effect of contrasting seasonality on different soil types.