Geraghty's move has ended speculation over his future
England international Shane Geraghty will leave London Irish at the end of the season after signing a two-year deal with Northampton.
Geraghty, 22, becomes the club's second big capture of the week after Newcastle captain and back-rower Phil Dowson signed on Wednesday.
The fly-half or centre joined Irish in 2004 and made his debut against Auch in the European Challenge Cup.
He made his Test debut against France in the 2007 Six Nations.
He also briefly appeared this season during England's victory over Italy but is currently out of action with a wrist injury.
"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.
"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."
Saints rugby director Jim Mallinder added: "Shane is an exciting, young, English talent, and I am delighted that he has signed for us.
"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."