Having played for the Junior All Blacks, Pisi has starred for Samoa on the sevens circuit and recently made his debut for the senior side in their Pacific Nations Cup win over Tonga.
"I want to experience something different," said Pisi.
"I was impressed by Jim Mallinder and the rest of the Saints coaches, and I've seen that it is a good environment, has a strong sense of community and is a club which takes care of players.
"Obviously the fact that George is doing well at the club made my decision easier.
"He's been enjoying his rugby and told me that the club is well organised with a tight squad, and everything that I've seen myself backs that up."
Director of rugby Jim Mallinder added: "We've been very excited by what we've seen from Ken.
"He is fast and physical and has good rugby awareness and handling skills.
"He is also versatile and can slot into a number of positions, which is important for us as we try to maintain a squad which can compete both in the Premiership and Europe while staying within the salary cap."