This dataset contains measured inherent optical properties (IOPs) of synthetic microplastic assemblages and contrasting natural seawater samples that have been additionally characterized in terms of particle size distribution and mass concentrations of particulate matter. The IOPs include spectral absorption and scattering coefficients of particles and the particulate phase function resolved with high angular resolution at a single wavelength (532 nm in vacuum). The particulate absoprtion coefficient was derived using the filter-pad method with a 25 mm diameter glass fiber filter (Whatman Grade GF/F) placed inside an integrating sphere and measured with a UV/VIS spectrophotometer. When available, the nonalgal (or detrital) absorption coefficient is also provided and was measured following pigment extraction with 95% methanol. The particulate scattering coefficient at 532 nm was determined through extrapolation and integration of the volume scattering function from LISST-VSF (Sequoia Scientific) measurements. The spectral characteristics of particulate scattering were either directly inferred from ac-s (Seabird Scientific) measurements or estimated from particle size distribution information. Particle size distributions (for diameters ~1–170 µm) were determined through inversion of near-forward scattering from the LISST-VSF measurements. For microplastic samples, LISST-HOLO2 (Sequoia Scientific) results of particle eccentricity are also provided. Additional polarized light scattering measurements of the m12 and m22 elements of the Mueller scattering matrix are also provided and were determined with the LISST-VSF instrument. All data has been processed and quality controlled. When appropriate, additional metadata is also provided including sampling location, time, and depth as well as temperature, salinity, dry mass concentration of suspended particulate matter (SPM), mass concentration of Particulate Organic Carbon (POC), and chlorophyll-a concentration (Chla). Headers within text files include further details and references for methodology. For more detailed information on LISST-VSF instrument and data processing, see Koestner et al. (2018) Applied Sciences and Koestner et al. (2020) Applied Optics. Note that "-999" is utilized as a placeholder for unavailable data within the txt file. Also note that all spectral absorption and scattering coefficients are provided as mass-specific coefficients indicating that they have been normalized by SPM. The dataset includes measurements of 66 samples, 11 of which are synthetic microplastic assemblages. The natural seawater samples in the dataset are from measurements near San Diego, California (refered to as SD16 in the filename, followed by the specific sample ID) and from the Beaufort Sea near Prudhoe Bay, Alaska (referred to as PB1819 and PB21 in the filename, followed by the specific station). The measurements of microplastic assemblages are indicated with MP21 in the filename followed by the shortname for the plastic material used to generate the sample. Further information regarding these data can be found in the related resources publications. The measurement files (provided as tabulated txt files) are formulated for incorporation into radiative transfer simulations of an ocean-atmosphere system with Flick v1.0 (doi: 10.5281/zenodo.10551539), a shell module which prepares inputs for the raditiave transfer software AccuRT. This dataset also includes simulation files (configuration files and results) for a series of simulations prepared to examine the remote sensing reflectance of an open-ocean background of typical composition with and without microplastics added at all 11 visible bands (400–709 nm) of the ocean and land color instrument (OLCI) on the Sentinel-3 satellite. Efforts were focused on the South Pacific Subtropical Gyre (SPSG), a region of very clear oligotrophic waters which has also been an accumulation zone for plastic debris. Further information regarding these simulations can be found in the related Limnology and Oceanography Letters manuscript and supplemental file (currently under review).