<p>Natural fiber–reinforced composites have gained increasing attention due to their sustainability and environmental benefits. Plant-based reinforcements offer a viable alternative to conventional synthetic composites; however, their susceptibility to moisture absorption remains a major limitation that adversely affects mechanical and thermal performance. To address this issue, the present study investigates the application of <i>Nelumbo nucifera leaf powder</i> (NNLP), a naturally hydrophobic material, as a surface coating for natural fibers to enhance moisture resistance. NNLP-coated composites were evaluated through water absorption, tensile, flexural, and impact tests, along with thermal conductivity measurements. Surface morphology and fracture behavior were examined using scanning electron microscopy (SEM) to assess fiber–matrix interfacial characteristics and failure mechanisms. Jute, hemp, and flax fibers embedded in an unsaturated polyester resin matrix were used to examine the effectiveness of the NNLP coating. The results indicate that NNLP coating reduced water uptake and improved tensile and flexural properties for all fiber types. Under distilled water immersion, tensile modulus increased by 50.35%, 19.42%, and 70.35% for jute, hemp, and flax composites, respectively, while corresponding improvements of 54.72%, 13.58%, and 71.95% were observed under seawater immersion. SEM observations revealed reduced void content and improved fiber–matrix interfacial bonding in coated composites, supporting the observed mechanical performance enhancement. Overall, NNLP coating demonstrates potential as a cost-effective and eco-friendly approach for improving the moisture resistance and durability of natural fiber composites.</p>