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Although recent research points to Mo as a candidate for anthropogenically induced contamination of natural waters, there has been little research on Mo contamination in river water. The Erdenet mine in Mongolia is the one of the largest Mo mines in the world. Our systematic temporal and spatial monitoring of Mo concentrations in river waters around the Erdenet mine from March to November 2018 showed that in the Khangal River, which is downstream of the Erdenet mining area, Mo concentrations exceeded 70 μg/L (the World Health Organization health-based guideline) during summer. The sources of the Mo in river water are groundwater used for cooling in the Erdenet power plant and seepage from mine tailing ponds. Mo concentrations had a positive correlation with the pH of the river water. Laboratory adsorption experiments confirmed that the pH dependency of Mo concentrations originates from the adsorption/desorption reactions of iron oxide in the river sediments. Mo concentrations in river water were still higher 10 km downstream of the contamination sources. Our observations provided field evidence of high solubility and mobility of Mo in river water that were caused by the lower distribution coefficients of Mo adsorbed on iron oxides in water at pH >8.