This dataset contains the total suspended solids (TSS) concentration of in situ water samples collected at selected sites across the Atchafalaya and Terrebonne Basins within the Mississippi River Delta (MRD) floodplain of coastal Louisiana, USA. The measurements were made during the Pre-Delta-X campaign in Spring 2015 and Fall 2016. The sampling sites spanned large and small channels at locations chosen to cover a representative range of suspended sediment concentrations from a variety of hydrodynamic and physical settings typically encountered across the basins. Water samples were collected by bottle just beneath the surface and stored on ice until filtering. The TSS concentration was calculated as the difference of the filter weight (before and after filtration) divided by the volume of sample filtered. Both TSS and in situ spectral reflectance measurements were collected at some sampling sites. Pre-Delta-X sampling focused on surface waters, where the TSS data are used as inputs for hydrodynamic models for sediment transport and to calibrate the model to convert AVIRIS-NG spectral reflectance measurements into TSS.