The 23-year-old outside back, the younger brother of Saints' player of the year George, arrives at Franklin's Gardens after five seasons with North Harbour in the ITM Cup.
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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
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Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.