As part of the U.S. Geological Survey's National Water-Quality Assessment Program, an investigation of the Yellowstone River Basin study unit is being conducted to document status and trends in surface- and ground-water quality. Efficient quantification of a variety of environmental and geographic characteristics of the study unit requires a digital map of the study-unit boundary that may be processed, together with other digital thematic maps (such as geology or land use), in a geographic information system (GIS). Digital boundary data for the Yellowstone River Basin study unit are included in this data release. The boundary follows drainage divides that were identified chiefly using 1:100,000-scale (50 m accuracy) hypsography.