Staff from the Cape Cod National Seashore, National Park Service (CACO NPS) began limited water-quality data collection in the Herring River Estuary as early as 1984. In 2006 a monthly sampling program was established for 14 sites to monitor nutrient concentrations, dissolved oxygen, and other constituents. This monitoring program is ongoing and has been supplemented with data from other CACO NPS projects and cooperative monitoring with the U.S. Geological Survey. With the anticipated restoration of the estuary, which has been diked off from the ocean for over 100 years, these data are critical in supporting the characterization of pre-restoration water quality. The data collected in the estuary from 1984-2017 have not been published and disseminated in one place to date. This data release is intended to address this issue and create a framework in which future data can be published. In preparation for publication, the data were compiled from several CACO NPS internal sources and were reviewed using very preliminary quality checks. The related USGS water-quality data are available in the National Water Information System (NWIS).