Modification of diatomaceous earth (DE) was performed using alkyltrimethoxysilanes of different chain lengths (C3, C8, C12, C16, and C18), and their resultant properties were determined. The thermal properties of these alkyltrimethoxysilane-treated DE powders were probed using thermogravimetric analysis and temperature-modulated differential scanning calorimetry, and the surface/porosity was studied using nitrogen adsorption and electron microscopy. Crystallinity of the hydrocarbon tails occurred when the chain lengths were C12 or larger, and the adsorbed hydrocarbon amounts were 1.6 mg/m2 or more. The wettability of functionalized DE-containing surfaces was studied using water contact angle measurements. At larger adsorbed amounts of 2.2 mg/m2 or more, the treated DE formed superhydrophobic coatings (with water contact angles ≥150°) with a polyurethane binder. These coatings required a minimum of 30% particle loadings, which allowed the DE particles to dominate the surface. At loadings larger than approximately 50%, there was a decrease in the contact angles corresponding to a reduction in roughness on the surface. Samples with adsorbed amounts less than 2.2 mg/m2 or chain lengths shorter than C12 were only hydrophobic. These results were in agreement with scanning electron microscopy and Brunauer-Emmett-Teller specific surface area and pore volume measurements.