This disclosure describes hydrogel three-dimensional printing kits, methods of three-dimensional printing hydrogels, and hydrogel three-dimensional printing systems. In one example, a hydrogel three-dimensional printing kit can include a particulate build material and a crosslinking agent. The particulate build material can include from about 90 wt % to 100 wt % of a polyhydroxylated swellable polymer. The crosslinking agent can include water and a crosslinker that is reactive with hydroxyl groups of the polyhydroxylated swellable polymer to crosslink the polyhydroxylated swellable polymer.