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With regard to the sustainability and biological origin of plastic components, regenerated cellulose fiber (RCF)-reinforced polymers are expected to replace other composites in the future. For use under severe conditions, for example, as a housing in the engine compartment, the resistance of the composites and the impact on the fiber and fiber-matrix adhesion must be investigated. Composites of bio-polyamide with a reinforcement of 20 wt.% RCF were compounded using a twin-screw extruder. The test specimens were manufactured with an injection molding machine and aged under conditions of high humidity at 90% r. H, a high temperature of 70 °C, and water storage using a water temperature of 23 °C for 504 h. Mechanical tests, single-fiber tensile tests (SFTT), single-fibre pull-out tests (SFPT), and optical characterization revealed significant changes in the properties of the composites. The results of the SFPT show that accelerated aging had a significant effect on the bio-polymer and an even stronger effect on the fiber, as the single-fiber tensile strength decreased by 27.5%. Supplementary notched impact strength tests revealed a correlation of the impact strength and the accelerated aging of the RCF-reinforced composites. In addition, it could be verified that the tensile strength also decreased at about 37% due to the aging effect on the RCF and a lowered fiber-matrix adhesion. The largest aging impact was on the Young’s modulus with a decrease of 45% due to the accelerated aging. In summary, the results show that the strengthening effect with 20 wt.% RCF was highly decreased subsequent to the accelerated aging due to hydrolysis and debonding because of the shrinkage and swelling of the matrix and fiber. These scientific findings are essential, as it is important to ensure that this bio-based material used in the automotive sector can withstand these stresses without severe degradation. This study provides information about the aging behavior of RCF-reinforced bio-based polyamide, which provides fundamental insights for future research.