Vilsack and his wife, Christie, planned to make the endorsement on Monday when the New York senator will be in the state, the officials said. They spoke on condition of anonymity because of the planned formal statement next week.
Clinton spokesman Mark Daley would not confirm Vilsack's endorsement, but did say the campaign had a major announcement planned for Monday in Des Moines.
Vilsack, who served two terms as Iowa governor, launched his own bid for president in November, but dropped out of the race on Feb. 23. He said his campaign could not raise enough money to continue.
Vilsack had been courted by most of the major Democratic contenders for the presidential nomination. In recent interviews, he had said he planned to endorse, but declined to give a timetable.
"At this point I'm not thinking about future steps in terms of other campaigns. I'm trying to make sure my campaign is ended properly," he said. "But ... I think the chances are good that I'll do that," he said earlier this month, reports AP.
Vilsack fielded calls from most of the Democrats running for president, including Clinton and Sen. Barack Obama. He also heard from former President Bill Clinton, former Vice President Al Gore and former Democratic National Committee chairman Terry McAuliffe.