These files contain data supporting all results reported in Fine et. al. Evidence for a Layered Structure of Titan's Plains Observed by the Cassini RADAR. In Fine et. al. we found: Undifferentiated plains are the most common terrain type on Titan, yet their composition and geologic history remain poorly understood. To better characterize their physical properties, we combined Cassini RADAR measurements from nadir altimetry and side-looking SAR modes. We analyzed these data using radar backscatter models, finding that the multi-angle radar response from plains across Titan is remarkably consistent. This uniformity suggests globally similar properties and formation processes, permitting aggregate modeling. Our analysis reveals that canonical single-layer scattering models fail to reproduce the observed backscatter, particularly the bright near-nadir returns captured by altimetry, which proved critical for model discrimination and accurate parameter estimation. Instead, a two-layer model is required to robustly fit the data across all incidence angles. Best-fit parameters indicate plains likely consist of a highly porous, low-density surface layer (effective permittivity ~1.33) that is exceptionally smooth at radar wavelengths (RMS slope ~2°), overlying a higher-density (effective permittivity > 2.7) and rougher buried substrate. This surface layer is likely less than one meter thick. The layered structure, along with the observed global uniformity and extreme flatness at multiple scales, is most consistent with long-term atmospheric deposition of organic particles (“tholin snow”), which are subsequently densified or buried, potentially during periods of different environmental conditions. The structure of the plains provides insights into organic processing and transport on Titan, potentially preserves a record of past environmental conditions, and informs landing safety assessments for future missions. Specifically, the hypotheses made by this paper will be testable by the Dragonfly mission to Titan.