A steady-state groundwater flow model, using MODFLOW-NWT, was developed to better understand groundwater flow in coastal Connecticut and adjacent areas of New York and Rhode Island. Aquatic systems in and around Long Island Sound (LIS) provide a variety of ecological and economic benefits such as flood and storm protection, water filtration, recreation, habitat for commercially and recreationally important fish and bird populations, and carbon sequestration. In some areas of LIS and in many embayments along the Connecticut coastline, aquatic ecosystems are degraded due to excess nitrogen from sources such as waste-water treatment plants, septic systems, and fertilizer. A substantial fraction of total nitrogen inputs to LIS are transported through the ground- water-flow system. Currently, the groundwater-flow system on the north shore of LIS is not well understood. Groundwater budgets, travel times, and discharge receptors have not been quantified, hindering water-resources management efforts focused on groundwater transported nitrogen. The groundwater model was used as a framework for understanding the groundwater component of flow to surface water in the HUC12 watersheds within the study area and to a subset of coastal embayments of LIS. The model was calibrated to groundwater levels and baseflow discharge from water years 1990 through 2019, long-term average stream altitudes, and a dry stream/flooded land metric. The model was used to calculate the components of the groundwater budget and to estimate groundwater travel times, using MODPATH particle-tracking, within HUC12 basins and embayment watersheds, as well as to estimate groundwater nitrogen loading and transport times within the Niantic River watershed. This dataset contains model inputs and outputs, post-processing python scripts, and PEST calibration input files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20215116).