Sustainable polysaccharides nanofibrils formed from chitin or cellulose are emerging biobased nanomaterials for advanced materials requiring high mechanical performance, barrier properties, for bioactive materials or other functionalities. Here, we demonstrate a single-step, waterborne approach to prepare additive-free flame-retardant and self-extinguishing, mechanical high-performance nanopapers based purely on surface-deacetylated chitin nanofibrils (ChNFs). We show that the flammability can be critically reduced by exchanging the counterions, e.g. to the phosphate type, using the respective acid providing electrostatic stabilization in the preparation of the ChNFs. This exchange renders beneficial elemental combinations of high contents of N/P in the final nanopapers, known to provide outstanding performance in halogen- and heavy metal-free flame-retardant materials. Full fire barrier nanopapers can even be obtained by hybridizing the ChNF with nanoclay. Comprehensive fire retardancy tests, including vertical and horizontal flame tests and microscale cone combustion calorimetry, as well as fire breakthrough tests elucidate excellent flame-retardant properties and high structural integrity when being burned. The intrinsic elemental composition of chitin, containing nitrogen, and the simple modification of the counterions to include phosphorus provides key advantages over related, but flammable nanocellulose materials, that often require significant chemical modifications and additives to become fire retardant. By activating a global food waste, this study presents a critical advance for bioinspired, green and mechanical high-performance materials with extraordinary flame-retardant and fire barrier properties based on sustainable feedstock, using benign water-based room temperature processing, and by avoiding heavy metals and halogen atoms in their composition.