"Kahn is a player who has attracted a lot of positive headlines and attention throughout his career," said Saints Director of Rugby Jim Mallinder.
"He has a superb set of skills and is a powerful and dynamic runner who will add to our options in one of the team's crucial positions.
"We are delighted that he has chosen to sign a long-term contract to play his rugby at Franklin's Gardens, and we're looking forward to welcoming him to the club in the summer."
Fotuali'i has won 13 caps for and in his two seasons at the Liberty Stadium he has played 39 times, including in the PRO12 final triumph over Leinster at the RDS.
Born in Auckland, Fotuali'i had been tipped for full All Blacks honours after making his mark in Super Rugby for the Crusaders, for whom he played 28 times.
Instead Fotuali'i chose to represent Samoa, and after a man of the match performance in his 2010 debut he went on to play in the 2011 Rugby World Cup.