Objectives. To estimate the population lacking at least basic water and sanitation access in the urban United States.Methods. We compared national estimates of water and sanitation access from the World Health Organization/United Nations Children's Fund Joint Monitoring Program with estimates from the US Department of Housing and Urban Development on homelessness and the American Community Survey on household water and sanitation facilities.Results. We estimated that at least 930 000 persons in US cities lacked sustained access to at least basic sanitation and 610 000 to at least basic water access, as defined by the United Nations.Conclusions. After accounting for those experiencing homelessness and substandard housing, our estimate of people lacking at least basic water equaled current estimates (n = 610 000)-without considering water quality-and greatly exceeded estimates of sanitation access (n = 28 000).Public Health Implications. Methods to estimate water and sanitation access in the United States should include people experiencing homelessness and other low-income groups, and specific policies are needed to reduce disparities in urban sanitation. We recommend similar estimation efforts for other high-income countries currently reported as having near universal sanitation access.