The 20-year-old's current contract with Scarlets had not been due to expire until the end of the 2014 season. However, following a request from both the player and the Saints, the Welsh club have agreed to release the winger this summer.
"Despite our desire to retain all our internationals within the region, we have to balance our available resource over the need to sustain a 50-deep player squad that can continue to compete during international periods," said Scarlets chief executive Mark Davies.
"Taking that into account, we made George the very best offer we could. However, we have to fully understand and appreciate that George's quite unique value in the rugby marketplace, possibly inside Wales, but certainly outside Wales, is considerably greater than the Scarlets as an independent business can reach."
Northampton's chief executive Allan Robson added: "Everyone in our rugby community recognises and respects what an outstanding job the Scarlets have done to support, develop and invest in George North as rugby player, along with many other internationals within their remarkably successful set-up.
"He is an exceptional talent and one of the most formidable back-three attacking players in world rugby - with so much more to achieve in his rugby career at just 20 years old. We look forward with great anticipation to what he can bring to his performances with Northampton Saints next season."
North has agreed a three-year deal with the Saints that will span until the end of the 2016 World Cup.