The Washington Post said Kaine's entry into the race gives Virginia Democrats a prominent candidate for what is expected to be one of the closest races in the country. Kaine also resigned his post as chairman of the Democratic National Committee.
On his Twitter feed that also included a link to a video on his record as a former Richmond, Va., City Council member and mayor as well as governor, Kaine said, "I'm running!"
The Post said Kaine's announcement ends the suspense that began when Webb said in February he would step down.
Former U.S. Sen. George Allen is running in the Republican Party to get his old seat back, and Tea Party leader Jami Radke is also trying for the Republican nomination, the Post said.
President Barack Obama and Democrats across the state lobbied Kaine to run after Webb's announcement.
The Post said Obama, who took Virginia in 2008, is likely to make the state an integral part of his re-election bid.