The utility of visible-light-mediated polymerization in tissue engineering has been limited due to the necessary use of potentially cytotoxic coinitiator and comonomer. Here, we report a visible-light-mediated thiol-ene hydrogelation scheme using eosin-Y as the only photoinitiator. Under visible light exposure, rapid and highly tunable step-growth gelation is achieved using PEG-norbornene and a model cross-linker dithiothreitol. In addition to investigating the gelation kinetics and properties of thiol-ene hydrogels formed by this new gelation scheme, we also report high cytocompatibility of these hydrogels using human mesenchymal stem cells (hMSCs) and pancreatic MIN6 beta-cells.