"I'm delighted to sign with Notts County today and I am genuinely excited by what the club is trying to achieve," said the 34-year-old ex-Arsenal and Tottenham Hotspur player.
Campbell, who won two Premier League titles and three FA Cups at Arsenal, was unveiled by England's oldest Football league club after his contract at Portsmouth expired in July.
County were bought last month by Middle East consortium Munto Finance and Campbell will be reunited with their director of football, former England manager Sven-Goran Eriksson.
"He's a great man," Campbell said of Eriksson. "He's managed all over the world, knows his football and he's a proper guy."
Campbell's move to a club in the bottom tier of the Football League is sure to raise eyebrows but the defender said all that glittered was not gold in the Premier League.
"Some Premier League clubs are all over the place behind the scenes. You'd be shocked," he said.
"They're not well-oiled machines, believe me."
County, formed in 1862 and founder members of the League in 1888, have not been in the top flight since 1992.
Campbell's former managers at Arsenal and Portsmouth, Arsene Wenger and Harry Redknapp, expressed surprise at the move.
"You cannot tell me that you expected him to go. In May did you expect him to go to Notts County? Certainly not," a baffled Wenger told reporters.
Tottenham Hotspur manager Redknapp added: "He's still in my opinion a top six central defender in the Premier League, so for Notts County to get him in that division is just incredible."
