ABSTRACT. One of the major, but often overlooked, challenges towards high end applications of nanocelluloses is to maintain their mechanical stability under hydrated conditions. As such, permanent covalent crosslinking or surface hydrophobization are viable solutions provided that neither processability nor interfibrillar bonding is compromised. Here we show an alternative based on physical crosslinking of nanofibrillated cellulose (NFC, also denoted as microfibrillated cellulose, MFC, and cellulose nanofibers, CNF) with chitosan for the preparation of transparent films. Transparency (~ 80 % throughout the visible spectrum) is achieved by suppressing aggregation and carefully controlling the mixing conditions: Chitosan dissolves in aqueous medium at low pH and under these conditions the NFC/chitosan mixtures form easily processable hydrogels.A simple change in the environmental conditions (i.e. an increase of pH) reduces hydration of chitosan promoting multivalent physical interactions between NFC and chitosan over those with water, resulting effectively in crosslinking. Films of NFC/chitosan 80/20 w/w show excellent mechanical properties in the wet state, with a tensile modulus of 4 and 14 GPa at low (0.5 %) and large (16 %) strains, respectively and an ultimate strength of 100 MPa (with corresponding maximum strain of 28 %). Remarkably, a strength of 200 MPa (with maximum strain of 8%) is measured at 50 % air relative humidity. We expect that the proposed, simple concept opens new pathways toward NFC-based material utilization in wet or humid conditions, a challenge that has remained unresolved.