The Curaçao international has agreed a two-year deal and links up with the squad in time for this week’s training camp in Austria.
Martina, 25, was born in Rotterdam and previously played for Erwin Koeman, Saints’ current assistant manager, during a two-year spell at RKC Waalwijk.
Executive Director of Football Les Reed commented: “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.”