This study examines the influence of water absorption on the mechanical properties of waste tire rubber particle-reinforced epoxy composites with varying rubber content. The composites were fabricated using the open mold solution casting method, and their tensile properties were assessed before and after water absorption in distilled water at room temperature. The findings indicated a progressive increase in water uptake from 0.69% to 1.11% and thickness swelling from 1.46% to 2.01% with higher rubber content, attributed to void formation and microstructural imperfections at the epoxy-rubber interface. The incorporation of 3 wt.% rubber particles resulted in the highest improvement in tensile strength (3.78%) and modulus (14.82%), demonstrating an optimal reinforcement effect. Following moisture exposure, the varying rubber content of composites for various tests showed a decline in tensile strength, modulus, and strain, suggesting that water absorption had a degrading effect on the material. Notably, composites containing greater amounts of rubber experienced less reduction in strain. This is attributed to the natural elastic property of rubber particles, which significantly helped counteract the negative effects of moisture. These results provide valuable guidance for adjusting rubber content to enhance the mechanical durability and resilience of the composites, thereby increasing their operational lifespan in moisture-rich environments.