Light-triggered chemistry has been used extensively in polymer cross-linking for the preparation of hydrogels. However, most photoclick reactions rely on the use of UV light that is easily absorbed by and damaging to cells and tissue. We report the utilization of red light-catalyzed oxidation of dihydrogen tetrazine for activation of tetrazine-norbornene inverse electron-demand Diels-Alder conjugation. The activation takes place rapidly under physiological conditions and was applied in polymer conjugation and cross-linking to form hydrogels. Gelation was demonstrated to occur behind a dermal tissue model with a thickness of 1 cm, and the cross-linking process is nontoxic to mesenchymal stem cells, allowing encapsulation of hMSC within the gel matrix.