<p>Incorporating phase change materials (PCMs) into textiles is a green technique to make smart fabrics that are more comfortable to wear in hot weather. In this study, innovative bio-based phase change material composites were created by esterifying fatty acids (myristic, palmitic, and stearic acids) with octadecanol and subsequently encapsulating them within a gelatin/pectin coacervated matrix. The composites were applied to cotton fabrics using a pad-dry-cure method, both before and after dyeing. Their thermal, morphological, and functional qualities were carefully tested. Differential scanning calorimetry (DSC) showed that octadecanoyl stearate/gelatin–pectin (1:2 ratio) composites had the highest enthalpy (≈ 261&#xa0;J/g) and the best duration index (DI), which meant they could store a lot of latent heat. SEM showed that the coatings were even and that the surface was smoother and more stable. FTIR confirmed that there were hydrogen bonding contacts between the composite and the cellulose substrate. After being washed several times, treated fabrics showed a big improvement in their capacity to regulate heat, comfort (Q-max, thermal resistance, and conductivity), and durability. Additionally, the order in which the dyeing was done affected thermoregulation. Fabrics that were colored first and subsequently treated had the best overall thermal and color performance. Mechanical testing indicated that the tensile strength was still there and that the crease recovery angles had gotten better, which proved that the structure was still strong. This study shows a scalable, entirely bio-based, and eco-friendly way to give cotton fabrics long-lasting thermoregulation. The breakthrough consists of the combined use of natural fatty acid–octadecanol phase change materials (PCMs) and gelatin/pectin biopolymer shells, providing a sustainable alternative to paraffin-based systems and enhancing functional textile creation for fashion, healthcare, and protective uses.</p>