The aim of this work was to reinforce bio-polyurethane (BPU) with cellulose nanofibers (CNF) to develop foams suitable for use as core in lightweight composite panels. The nanofibers were prepared using mechanical grinding of bleached carrot juice residue, a cheap and energy efficient process. The prepared foam properties were studied and compared to neat BPU foam properties. The results showed that the CNF reinforced foam had better mechanical properties compared to the neat bio-PU foams and the addition of cellulose nanofibers deceased the cell size and open cell content. Then the foam suitability for composite use was evaluated by manufacturing lightweight sandwich panels using vacuum infusion process with CNF reinforced BPU foam core. Kraft paper was used as a skin and epoxy resin was used as adhesive resin and the composite laminates were prepared using vacuum infusion technique. These nanofiber reinforced core materials resulted in sandwich panels with improved mechanical properties. X-ray tomography showed that the resin did not penetrate into the core but only the foam surface layer. Moreover, the results were evaluated in a material selection process by means of minimizing merit indices. A trend in the behaviour of compressive properties of the foam and flexural properties of the sandwich panels could be established. The addition of small amount of cellulose nanofibers to BPU based foams is leading to a foam which have similar properties as commercial rigid PU foams. © 2017 International Committee on Composite Materials.