Biobased polymers and composites have gained increased global attention due to their abundant availability, renewability and biodegradability. Natural fillers, e.g. cellulose-based fillers, improve mechanical properties of biopolymers extending their application range, while keeping the ecofriendliness of the materials. Mowing towards engineering applications, requirements imposed to materials’ durability under environmental impact are increasing. Variations of ambient humidity and temperature could essentially reduce service lifetime of biobased polymer composites. This study is focused on hydrothermal degradation of poly(butylene succinate) (PBS) filled with nanofibrillated cellulose (NFC) up to 50 wt.% aimed to identify the most efficient PBS/NFC composition, while keeping a reasonable balance between the reinforcement effect and accelerated degradation inherent for most natural fillers. Water absorption and its effect on the structure, thermal, mechanical and thermomechanical properties were studied. Reasonably high reinforcement and adhesion efficiency is obtained for PBS/NFC composites that is maintained after their hydrothermal ageing. Water absorption capacity and diffusivity increased significantly with NFC content in PBS. Degradation of the mechanical properties is the higher, the higher NFC content in the polymer is. PBS filled with 20 wt.% of NFC is identified as the most efficient composition, for which negative environmental degradation effects are counterbalanced with the positive reinforcing effect.