This dataset presents the processed sidescan mosaic for Fire Island National Seashore. The acoustic sonar survey was conducted using a EdgeTech 6205, a Multi-Phase Echo sounder system. This combined bathymetry and dual-frequency sidescan sonar system is optimized for shallow water surveying. The sonar system was bow-mounted to a 28-ft pontoon survey vessel customized for shallow water surveying, including having a draft of less than one foot. Data was collected using GeoDas software developed by Ocean Imaging Consultants (OIC) and monitored topside in real-time to ensure quality of data and full-coverage sidescan was being achieved. The data were collected in association with an Applanix POS MV system to assure positional accuracy and to correct for vessel motion (pitch, roll, heave). The survey was designed to acquire full-coverage sidescan data and partial coverage bathymetry data. As such, the survey was composed of parallel track lines with line spacing of 35 to 50 m and a sonar swath range of 50 m (25 m per side) to ensure overlap with adjacent lines. The raw sidescan and bathymetry records were processed using OIC CleanSweep software (Version 3.8.0, Build #263). All of the acoustic data were examined manually to confirm quality and accuracy. For sidescan, processing followed standard techniques of bottom-tracking the data and then applying angle-varying gains (AVG) and look-up tables (LUT) as necessary to correct for water column returns, arrival angle, and contrast to produce color-balanced sidescan sonar images. The native images are displayed as an inverse gold color scale, with pixel values ranging from zero (dark gold) to 255 (white). The lighter pixels indicate hard acoustic returns and represent the presence of hard surficial sediments (e.g. coarse sand, cobbles, and boulders), whereas darker pixels represent the presence of soft sediments, which tend to absorb sound to a greater degree. The horizontal coordinate system was set to UTM Zone 18N and sidescan data was processed to 25cm pixel resolution.