The 23-year-old Uruguay international, who made two appearances at the World Cup in Brazil this summer, has reportedly headed north to the KC Stadium after failing to start a game for Saints so far this season.
Ramirez joined Southampton from Bologna for £12million in August 2012 and has made 50 appearances for the club and scored eight goals.
However, he has featured only twice as a substitute under new manager Ronald Koeman this term and the Dutchman looks set to allow him to join the Tigers on a temporary basis.
Ramirez will join his fellow Uruguayan Abel Hernandez at Hull after the striker completed his club-record move from Palermo earlier on Monday.
Tigers manager Steve Bruce told his club's official website: “Gaston significantly improves our attacking options.
"I’m delighted to welcome him to the club. He is yet another quality addition as we look to build on what we achieved last season.”