The 31-year-old was a member of both the 2011 and 2015 World Cup-winning teams - and has 106 test caps to his name.
Franks will join older brother Ben and former All Black prop Ben at Northampton - and says the move presents a great opportunity to test himself in a new rugby environment, and reconnect with family there.
Franks has been named to start for the Crusaders in their Super Rugby opener tomorrow night against Blues in Auckland.
