"I am delighted to be taking up the new challenge that has been offered to me by Northampton Saints and to work with the coaching team who I know from my involvement with the England Under 21 team," said Geraghty, who made his senior England debut against France in 2007 and last appeared in national colours in the win over Italy earlier this year.
"This opportunity will allow me to enjoy being part of the club's future ambitions and also enable me to hopefully force my way back into the England team."
In their first season back in the top-flight, Northampton currently sit eighth in the Guinness Premiership standings and director of rugby Jim Mallinder believes Geraghty is the kind of player who can help them push on to greater things.
"Shane is an exciting, young, English talent and I am delighted that he has signed for us," said Mallinder.
"Although he is still young, he has played at all levels of the game, including England internationals and a Heineken Cup semi-final, as well as the Guinness Premiership.
"This experience, existing talent and potential to improve still further make Shane an excellent addition to our squad.
"We want players who can help Northampton Saints achieve our ambitions to become one of the top clubs in England and Europe over the next few years, and Shane is one such player."