This dataset provides measurements of in situ remote-sensing reflectance (Rrs; per steradian) of surface water across Atchafalaya Basin, southern coastal Louisiana, USA within Mississippi River Delta (MRD) floodplain. The in situ spectral reflectance measurements were made during the Pre-Delta-X campaign in Fall 2016 (October 14 to 2). Hand-held spectrometer measurements were collected from a boat at 35 locations selected to represent a range of suspended sediment concentrations and properties from a variety of hydrodynamic and physical settings typically encountered across the Atchafalaya basin. These 35 spectral reflectance measurements were collected at 24 unique sites that coincide with measurements of total suspended solids (TSS). The data serves two main purposes, to ground-truth the remote-sensing reflectance derived from NASA's Airborne Visible-Infrared Imaging Spectrometer - Next Generation (AVIRIS-NG) instrument, and to calibrate and validate algorithms for the retrieval of TSS from AVIRIS-NG.