"Hillary Clinton is the champion working families deserve in the White House," said Gephardt, the former Missouri congressman. "... She's got the strength and experience to deliver the kind of change America wants."
The endorsement came as Clinton finished up a four-day campaign swing through Iowa, which hosts the first voting contest Jan. 14. Former President Clinton campaigned with his wife for the first three days of the Iowa trip.
Gephardt won the Iowa caucuses in his first presidential bid in 1988. But his underfunded campaign lost steam and he dropped out of the race in March after doing poorly in that year's "Super Tuesday" contests.
Gephardt banked heavily on winning Iowa again during his second presidential bid in 2004. He finished fourth that year after engaging in a nasty battle with Howard Dean, and ended his campaign the next day.
Gephardt has worked as a lawyer-lobbyist in Washington since retiring from politics in 2005.