He is currently plying his trade with the French champions after being drafted in as a medical joker for the final third of the French domestic season.
Pisi, who can also play full-back finished last season as Samoa's first choice centre, making his debut in the Pacific Nations Cup in June and starting against Ireland, England and Scotland during the Autumn Internationals.
"It's an awesome club," he said.
"I've admired the people who have been through it. It's a great family club and it's a great place for me to start my own family over there. I've been watching how the Saints have been going. They've been playing well and come along from last year and I feel that the team is building every year for higher honours.
"Playing against Chris Ashton and Ben Foden in the Autumn was awesome and it'll be exciting to get the chance to play with them in England. Taranaki have been good to me and it's hard to leave them, but it's an opportunity for me to have a new life in Northampton with my fiancé Jessica.
"Hopefully playing at Clermont will be good for me. The more rugby I play the better I get, so when I come back for Taranaki, then hopefully the World Cup with Samoa, I'll be able to hit the ground running when I get to Northampton."
Director of rugby Jim Mallinder says that Pisi is a good addition to the Saints squad.
"George has improved a lot over the past year or so and we were impressed with what we've seen of him playing for Samoa in the Autumn and Taranaki in the ITM Cup," he said.
"He's a strong runner who likes to play with the ball in hand and although he played for Samoa at centre he is comfortable in a number of positions, which will be good for our squad as a whole."