Water security is one of the biggest challenges of the 21st century. Understanding context-specific challenges and opportunities around this issue is key to improving water systems globally. This paper explores the current state of urban water security in Ulaanbaatar, Mongolia’s capital city. Ulaanbaatar is home to more than 40% of the country’s population and 60% of its national GDP. The city is located in the Tuul River basin and relies almost entirely on groundwater aquifers of the Tuul River for its supply of clean drinking water. In recent years, socio-economic stressors resulting from rapid urbanisation and environmental pressures have intensified the levels of degradation of the Tuul River and intensified the risks of water insecurity for the population of Ulaanbaatar. This study combines quantitative and qualitative methods to provide a preliminary assessment of water security at the urban level. This paper presents an urban water security index for the dimensions of water supply and sanitation, water productivity, water environment, water-related disasters and water governance. The findings and discussion are supplemented with information from key informant interviews. This paper concludes by highlighting the important limitations that exist in terms of data availability for an urban scale assessment. The results suggest that important water security inequalities also prevail within the Ulaanbaatar city-region itself which are often masked by using average scores in assessments of this sort.