Chitin hydrogel has been recognized as a promising material for various biomedical applications because of its biocompatibility and biodegradability. However, the fabrication of strong chitin hydrogel remains a big challenge because of the insolubility of chitin in many solvents and the reduced chain length of chitin regenerated from solutions. We herein introduce the fabrication of chitin hydrogel with biomimetic structure through the chemical transformation of chitosan, which is a water-soluble deacetylated derivative of chitin. The reacetylation of the amino group in chitosan endows the obtained chitin hydrogel with outstanding resistance to swelling, degradation, extreme temperature and pH conditions, and organic solvents. The chitin hydrogel has excellent mechanical properties while retaining a high water content (more than 95 wt.%). It also shows excellent antifouling performance that it resists the adhesion of proteins, bacteria, blood, and cells. Moreover, as the initial chitosan solution can be feasibly frozen and templated by ice crystals, the chitin hydrogel structure can be either nacre-like or wood-like depending on the freezing method of the precursory chitosan solution. Owing to these anisotropic structures, such chitin hydrogel can exhibit anisotropic mechanics and mass transfer capabilities. The current work provides a rational strategy to fabricate chitin hydrogels and paves the way for its practical applications as a superior biomedical material.