These data are high-resolution seismic reflection profile data of the seafloor offshore of Massachusetts, from Nahant to Gloucester. Approximately 1,175 kms of seismic reflection profile data were collected using a Knudsen 320b chirp system Data were processed using SIOSEIS (Scripps Institute of Oceanography) and Seismic Unix (Colorado School of Mines) to produce segy files and jpg images of the profiles. Data were then imported into Landmark SeisWorks, an interactive computer system where horizons were interpreted and digitized to calculate depth to reflectors below the seafloor. Interpreted depth to reflectors were calculated, exported every 5 shots, and then imported to ArcGIS for interpolation into a 50-m raster grid. These data were used to generate a sediment isopach map, which shows the total thickness of sediment that overlies bedrock