Forster has made 134 appearances for the Saints since joining from Celtic in 2014.Southampton goalkeeper Fraser Forster is set to sign for Premier League rivals Tottenham Hotspur on a free transfer.
The England international's contract expires at the end of next month, which allows him to move clubs as a free agent.
BBC Sport understand that the 34-year-old has already undergone a medical and the deal will be announced soon.
Forster will provide back-up to Spurs captain Hugo Lloris.
Lloris' current deputy Pierluigi Gollini is on loan from Serie A side Atalanta, but his future is unclear. Spurs have the option to buy him, but in recent days he revealed he would be happy to return to Italy.
Former Newcastle and Celtic keeper Forster joined Southampton in 2014 and made 156 appearances for the south coast club. He has been capped six times by his country.
