Ant–plant interactions have mainly been considered as a protection mutualism where ants increase plant performance through protection from herbivory. However, host plants may also benefit from nutrients deposited by ants. Nitrogen limits the plant growth in most terrestrial ecosystems and the nutrient exchange between ants and plants may be an important mechanism operating in ant–plant interactions. In this study, we quantified the exchange of macronutrients (carbon and nitrogen) between ants and plants, using the Asian weaver ant Oecophylla smaragdina as a model species in a mango agroecosystem. A method was developed with which the amount of nitrogen retrieved to their host trees could be predicted by the trail density of O. smaragdina. Ant nutrient consumption was calculated based on data on O. smaragdina abundance and per capita consumption rates obtained in laboratory colonies. On a yearly basis, the influx of nitrogen to the host tree, originating from captured prey, averaged 14.4 (range 8.0–46.4) kg N ha−1 y−1. The loss of carbon from the host tree due to ant consumption of exudates from nectaries and tended homopterans averaged 278.1 (range 149.3–939.9) kg C ha−1 y−1. O. smaragdina may provide their host plant with a significant source of nitrogen albeit a substantial amount of carbon is consumed from the host plant. This study reveals that the flow of nutrients between ants and plants may play a critical and underestimated role in ant–plant mutualisms.