Rubber tree plantations (Hevea brasiliensis) are expanding into marginal areas with low soil fertility and long dry seasons with a high risk of soil erosion and drought damage to trees. Introducing an N2-fixing legume cover crop in rubber plantations may reduce runoff and soil erosion as well as increasing the availability of nutrients but may also increase competition for water. This study quantified the effect of the legume cover crop Pueraria phaseoloides on N, P and K nutrition, water status and growth of young rubber trees (three years old in 2007) over a four year period (2007–2010). The plantation was located on a toposequence with a range of soil depths and water storage capacities in northeast Thailand. The legume aboveground biomass production and its nutrient content and decomposition rate were measured and the N2 fixation was estimated using the abundance of 15N (d15N) in the legume. Measurements were taken of the tree stem girth and height and tree leaf predawn water potential, nutrient content and greenness. The transfer of N2 fixed by the cover crop to the trees was estimated using d15N in the tree leaves. The annual biomass production of the legume was 8 Mg ha-1 year-1 and the N accumulation by the legume was 250 kg N ha-1 year-1. The natural abundance method applied to the aboveground components of the legume gave N2 fixation rates varying from 85 to 93% depending on the year. The leaf d15N was similar in the three non-legumes (H. brasiliensis, Vetiveria zizanioides and Praxelis clematidea) used as reference plants for estimating the N2 fixation. The higher level of N and the much lower leaf d15N values for the rubber trees intercropped with P. phaseoloides, compared to rubber trees growing without a legume cover crop, showed that there was a relatively high transfer of fixed N from the legume to the trees, varying from 39% to 46% of tree leaf N depending on the year. Neither N2 fixation nor N transfer varied significantly along the toposequence. At