Whitehouse, 49, made the announcement at his home beside his wife and children less than a week after fellow Democrat Rep. Patrick Kennedy said he would not seek the seat.
Chafee has started raising money for an expected re-election campaign but has not announced his candidacy. Democratic Secretary of State Matt Brown (search) is the only other person who has officially entered the 2006 race.
Whitehouse was attorney general from 1999-2003. He made a failed bid for the Democratic nomination for governor in 2002.
As the state's top law enforcement officer, Whitehouse targeted former manufacturers of lead paint with a groundbreaking lawsuit claiming they created a public nuisance by making paints that poisoned thousands of children. It ended in a mistrial, and the case is still being litigated by Whitehouse's successor.
He angered minorities for his investigation into the death of Sgt. Cornel Young Jr., who was shot by two fellow officers in January 2000. The officers were cleared of any wrongdoing.