Martina, a right-back who can also play centre-half, will link up with the squad in time for this week’s training camp in Austria and will provide added competition for fellow summer recruit Cedric Soares following Nathaniel Clyne’s move to Liverpool.
Southampton are also still hoping to lure Toby Alderweireld back to the South Coast after a successful loan move last season but Tottenham appear to have edged ahead in the race for the Belgian international.
Curaçao international Martina, 25, was born in Rotterdam and previously played for Ronald Koeman’s brother and current Saints’ assistant manager Erwin Koeman, during a two-year spell at RKC Waalwijk.
Executive Director of Football Les Reed told the club’s official website: “Cuco is a versatile and experienced defender who will add strength and depth to our squad ahead of the coming season.
“Injuries to Ryan Bertrand and Bevis Mugabi highlighted the need to bring in another defender, and Cuco is the perfect solution.
“He is another hungry player who is excited about coming to our club, and will provide healthy competition for places in our defence.”
Martina added: “I’m very happy to be here at a big club like Southampton. I’ve followed Southampton now for two years and I see they are now a big club.
“I know Ronald Koeman and Erwin Koeman – I worked with Erwin for one year and everything was good. He knows my qualities and I know what he wants.
“It was a dream for me to play in the Premier League and finally I can play now. I hope to play many games and show the fans who I am.”
Southampton have already signed Dutch goalkeeper Maarten Stekelenburg, Portuguese defender Soares and Spanish striker Juanmi as Ronaldo Koeman strengthens his squad ahead of the new Premier League season.