Tropical Usumacinta River is the tenth largest in North America and Mexico's principal river. Diverse and increasing anthropogenic activities (e.g., land-use change, agriculture, urban development) occur along the river and most likely alter its water quality. We used inverse geochemical models (PHREEQC code) and chemical diagrams to determine the water chemistry in the mainstream and principal tributaries, the principal geological and anthropic causal factors, and the effects of tropical seasonality. The dominant chemical water type in the river is Ca-Mg-SO 4 -HCO 3 in the dry season, while Ca-Mg-HCO 3 -SO 4 in the rainy season. The water-rock interaction processes in the dry season are: 1) gypsum, halite, and sylvite precipitation, and Mg2+ to Na+ ion exchange in the middle basin; 2) calcite, kaolinite, and SiO 2 (a) precipitation, CO 2 , dolomite, gypsum, halite, sylvite, and K-feldspar dissolution, nitrification, and Ca2+ to Na+ ion exchange in the lower basin. In the rainy season, the processes are: 1) plagioclase and sylvite precipitation, calcite, dolomite, gypsum, halite, kaolinite, and K-feldspar dissolution in the middle basin; 2) kaolinite precipitation, dolomite, gypsum, halite, K-feldspar, and sylvite dissolution, and nitrification, in the lower basin. The model outcome reveals the water-rock interactions that control the chemical composition in the river are mainly the carbonate and secondly the silicate weathering in the dry season. In contrast, in the rainy season, the weathering of carbonate and silicate rocks have the same weathering proportion. At the same time, the model of the dry season reveals the influence of saline water intrusion in the lower basin. The effects caused by agricultural activities as nitrification are evident in the lower basin of the dry season. In contrast, it is not present in the rainy season most likely associated to the river flow dilutes the nitrates. The results of this research are slightly different from other tropical rivers with similar geology, with a predominance of Ca2+ and Mg2+ cations, and HCO 3 - and SO 4 2- anions. This investigation is the first to present the major ions composition, chemical water types, and water-rock interactions in the tropical Usumacinta River. The methodology herein used proved to be useful to understand the chemical changes in rivers with various geological materials that could be applied in other tropical and temperate rivers. • The first study showing the major ions, chemical water types, and water-rock interactions of Usumacinta River. • Tropical seasonality defines the chemical changes with preferential dissolution of carbonate and silicate minerals. • Saline water intrusion in the river-mouth zone increases Cl-, Na+, and SO 4 2- in the dry season. • The main hydrogeochemical processes in the mainstream are provided for both seasons with geochemical models. • Anthropic pollution as NO 3 - enrichment was found related to agricultural activities in the lower basin of dry season.