Both clubs announced the deal on Monday evening, with the Saints needing to make a move after Forster damaged his left patellar tendon in March - an injury which he is expected to take up to a year to recover from
Stekelenburg, capped 54 times by his country, spent last season on loan at Monaco following Fulham's relegation to the Sky Bet Championship.
Stekelenburg, 32, made only four appearances for Monaco as he failed to dislodge first-choice Danijel Subasic, and he is now hoping to find regular football at St Mary's
"I'm very pleased that I've joined," he said on Southampton's website.
"It's a big, nice club that will play European football - and I can play
Last year I didn't play a lot, so it's an opportunity for me
"The last two years weren't the best of my career
As they say in England: the best is yet to come."
Stekelenburg joined Fulham from Roma in 2013, but could not prevent their drop into the second tier
He began his career by coming through the ranks at Ajax and became Holland's first-choice goalkeeper after Edwin van der Sar's retirement ahead of the 2010 World Cup, but he has not featured for the national team since October 2012.
Southampton are hoping that his experience can help them at home and in Europe this coming season.
"Maarten is a goalkeeper with a wealth of top-level experience who will prove to be an important addition to the squad at this early stage of pre-season," executive director of football Les Reed said
"While Fraser Forster's rehabilitation is progressing well, adding Maarten to our squad alongside Kelvin Davis and Paulo Gazzaniga will give us strength in depth for what will be a busy season.
"Maarten enjoyed tremendous success in Holland with Ajax and on the international stage, playing in the 2010 World Cup final
His experience will be invaluable to us, especially with the arrival of European football.
"There was strong competition for Maarten's services, so we are very pleased he has committed to our cause."
Source : PA