"We say thank you to Illinois for putting us over the top in the delegate count, and for helping us to achieve our goal," Kerry said after the primary election in Illinois state that took him past the necessary 2162 delegates to win the formal nomination.
Kerry has already embarked on a fierce campaign against Republican President George Bush for the 2 November election.
He has been the de facto Democratic contender since sweeping nine of the 10 states, decided in the 2 March Super Tuesday votes. His main rival, Senator John Edwards, withdrew immediately after.
"This night marks the opening of the general election debate about the direction of our country," Kerry said.
"America deserves more than a debate full of attacks and distortions. The times are too serious and the challenges we face too great," he said.