Santorum's illness forced him to cancel two appearances scheduled for Monday in Iowa.
Virginia Davis, a spokeswoman for Santorum's political action committee Patriot Voices, told WHO-TV, Des Moines, Santorum was suffering from a "gastro-intestinal illness and dehydration."
"He is feeling better today but remains in the hospital," she said. "He is expected to resume a full schedule later this week and looks forward to returning to Iowa soon."
Davis told The Hill Santorum became ill while "traveling in South Carolina on Saturday." She did not say where he was hospitalized.
Santorum was to have spoken at a fundraiser for the Iowa Faith & Freedom Coalition in Urbandale, Iowa, and visited the John Paul II Medical Research Institute for a stem cell research event in Des Moines, WHO-TV said.