The midfielder has failed to hold down a regular place in the Saints' starting XI since joining the club from Bologna in 2012, and after making just 23 Premier League starts will now spend the 2014-15 campaign at the KC Stadium.
“Gaston significantly improves our attacking options and I’m delighted to welcome him to the club,” Steve Bruce told the club’s official website.
“He is yet another quality addition as we look to build on what we achieved last season.”
Ramirez becomes the third Tigers signing on transfer deadline day following the permanent signings of Abel Hernandez and Mohamed Diame.
Goal understands the club are also looking to push through a deal to bring Newcastle winger Hatem Ben Arfa to Humberside.