The 17-year-old was close to joining Spurs during the last transfer window but stayed at St Mary's and helped George Burley's side into the Coca-Cola Championship play-offs.
Manchester United had expressed an interest in the Wales teenager in January, with boss Sir Alex Ferguson admitting Spurs were ahead of them in the race to him.
Spurs have agreed an initial fee of £5million which could be doubled depending on Bale's progress.
Although Bale has been linked with United, and also Arsenal, since Southampton's promotion hopes ended, Spurs have moved quickly following the completion of the season as they did a year ago when signing Dimitar Berbatov.
"I want to play in the Premiership, Tottenham is a club pushing forward and I want to be part of its future," said Bale.
Bale put himself in the spotlight at the start of the season with a stunning free-kick against Derby, followed up with another set-piece against Coventry three days later.
He is the youngest player to be capped by Wales and despite being deployed at full-back so far in his fledgling career, has been tipped to emulate Ryan Giggs if he is developed into a winger.
Spurs boss Martin Jol is looking to strengthen the left flank of his team, so Bale could be played in defence or midfield.
Bale's instinct to push forward made him Southampton's most influential attacker as they sought promotion.
Saints boss Burley wanted to keep Bale at the club for another year to ensure he gets regular first-team football.
Bale's former Southampton room-mate, Theo Walcott, has seen his chances limited since moving to Arsenal but Bale is expected to challenge for a regular starting place at White Hart Lane, particularly with Spurs' problems on the left during the season.
The move is also consistent with Spurs' policy of buying the best young British talent when possible.
Jol had six players in the England B squad for the friendly against Albania, while Tom Huddlestone is in the Under-21s squad for the European Championships next month in Holland.
Sporting director Damien Comolli added: "We have been following Gareth for a long time now and have been very tenacious, so we are absolutely delighted to get him here.
"He is a player of great quality and, as we always say, our aim in each window is to improve the quality of the squad.
"Gareth has great physical attributes, he is tall, good in the air and has a fantastic left foot. He is very good on set pieces, he is clever on the ball and his passing selection is excellent.
"He has the ideal combination to become a top, top player and he is only 17."
Sporting Life