Stormwater Action Monitoring (SAM) is a collaborative monitoring program between western Washington municipal stormwater permittees, state and federal agencies. SAM’s role is to use the results of regional monitoring and focused studies to inform policy decisions and identify effective strategies to improve stormwater management in the Puget Sound region. The SAM program includes status and trends monitoring of water quality, stream biota (macroinvertebrates, algae), and stream habitat to measure whether conditions are getting better or worse and identify patterns in healthy and impaired Puget Lowland streams. To meet this objective, a framework of fundamental geospatial data was required to develop physical and anthropogenic characteristics of the study region, sampled sites and corresponding watersheds, and riparian zones. This dataset represents the watershed boundaries for the 105 sites sampled for the SAM small stream study, and is one of the four fundamental geospatial data layers that were developed for this study. In addition, watershed boundaries for 16 reference sites in the Puget lowlands sampled by the Washington State Department of Ecology’s Ambient Biological Monitoring program from 2010 to 2015 were included in this analysis to provide a regional context for the SAM study. In total, watershed boundaries for 121 total sites are provided in this data release.