This study demonstrates an approach for incorporating cellulose nanofibrils (CNFs) into polyurethane (PU) using a solvent-free masterbatch consisting of 30% CNFs and 70% polyol, where the latter component functions as a carrier. This is the highest concentrated re-dispersible CNF-polyol system described to date. CNF addition produced strong shear-thinning behaviour, reduced the temperature sensitivity of viscosity, and maintained network integrity under conditions representative of PU curing. Power-law and Arrhenius modelling indicated that at low-to-medium shear rates the CNFs dominate rheological response, but the polyols rheological profile dominate at high shear rates when the fibrils occupy less space due to fluid flow alignment. Oscillatory tests showed increased elasticity and the transition from a viscous fluid toward gel-like behaviour at around 2% CNF loading. Cured CNF-PU composites exhibited increased stiffness, with microscopy revealing mechanically engaged fibrils within the fracture surfaces. Overall, CNFs provide an effective bio-based route to rheology modification PU systems, highlighting their potential in high-performance coatings and adhesives