||Hydrogels are broadly used in various biomedical applications due to their good biocompatibility and ability to mimic biological environments. An interesting class of a hydrogel, called as “injectable hydrogel”, forms networks in a body. However, such conventional hydrogels mostly rely on mild chemical reactions, temperature/pH sensitive materials, or self-assembling molecules, which lack controllability over reaction and result weak and unstable hydrogels. In our study, we developed the novel system for hydrogel formation in vivo with good controllability using tissue penetrating NIR light. Hydrogels are formed via a photothermal polymerization process, in which Au nanorods convert NIR light to localized heat that triggers radical polymerization of diacrylated PEG. We accomplish the gelation with mild power of 785 nm NIR light with spatiotemporal control over the reaction. This novel photopolymerization platform will open a new window for in situ hydrogelation in vivo.