Staff of Mr Edwards, who is currently on a visit to Louisiana, let slip his plans by accidentally launching his campaign website a day early.
The former senator, 53, is the third man to announce his bid for the Democrat nomination for 2008.
His website carried the logo "Tomorrow begins today" before being shut down.
Mr Edwards' adviser, Jennifer Palmieri, said: "Better a day earlier than a day late."
Mr Edwards will formally reveal his plans on Thursday as he visits the rebuilding efforts in New Orleans that have followed last year's Hurricane Katrina.
He joins outgoing Iowa Governor Tom Vilsack and Ohio Representative Dennis Kucinich in the running for the Democrat nomination for 2008.
Mr Edwards has headed a University of North Carolina poverty research centre since the 2004 defeat.