: This study focuses on addressing the issue of moisture ingress in polymer composites, a factor that often compromises structural performance and limits service life, especially in humid or outdoor conditions. A hybrid composite was fabricated using pineapple fibre as reinforcement and waste plastic particles as filler, aiming to improve resistance to water absorption. Composites with varying fibre and filler proportions were prepared and tested in accordance with ASTM standards. Statistical evaluation using analysis of variance (ANOVA) indicated that pineapple fibre content exerted the most significant influence on water absorption (p < 0.001), followed by waste plastic content (p = 0.005), whereas the chemical surface treatment showed minimal impact (p = 0.083). The combination of 5% pineapple fibre, 15% waste plastic, and sodium hydroxide treatment yielded the lowest water uptake over both 4 hr and 24 hr immersion periods. These results highlight the potential of incorporating recycled plastic as effective filler for producing moisture-resistant, sustainable composites. The findings can guide the development of eco-friendly materials for use in applications such as construction panels, automotive parts, and other components requiring enhanced durability in moist environments.
