Lignin serves as a binder that forms strong matrices of the cell walls of wood. However, it has many photolabile chromophore groups that create a monotonic brownish color and make wood susceptible to photodegradation. Herein, a new strategy is reported for modifying lignin using an in situ, rapid, and scalable process that involves the photocatalytic oxidation of native lignin in wood by H₂O₂ and UV light. The reaction selectively eliminates lignin's chromophores while leaving the aromatic skeleton intact, thus modulating the optical properties of wood. The resulting “photonic wood” retains ≈80% of its original lignin content, which continues to serve as a strong binder and water‐proofing agent. As a result, photonic wood features a much higher mechanical strength in a wet environment (20‐times higher tensile strength and 12‐times greater compression resistance), significant scalability (≈2 m long sample), and largely reduced processing times (1–6.5 h vs 4–14 h) compared with delignification methods. Additionally, this in situ lignin‐modified wood structure is easily patterned through a photocatalytic oxidation process. This photocatalytic production of photonic wood paves the way for the large‐scale manufacturing of sustainable biosourced functional materials for a range of applications, including energy‐efficient buildings, optical management, and fluidic, ionic, electronic, and optical devices.