The 26-year-old forward, who was laying concrete in California just 10 months ago, has won rave reviews for his performances since arriving from San Francisco Golden Gate, winning man of the match awards in the Aviva Premiership Rugby win over Harlequins and Heineken Cup home victory against Castres.
In his first season at Franklin's Gardens Manoa has already notched 23 appearances in the black, green and gold as either a second row or number eight, missing just the one match in all competitions so far.
The Concord-born US international says he had no hesitation in signing on again to stay in the East Midlands.
"I'm blessed to have been able to sign for another three years with the Saints," he said. "I'm very happy to be here. The club's been great to me and my family so that's why I decided to stay here and sign this contract.
"Coming from the Super League in the States and jumping straight into the pros was big for me at the time. I've been learning a lot and at this stage I'm pleased with how I've been playing.
"Every single game I've played in has been a highlight for me, and knowing I'll be here for three more years is a good feeling for me and my family."
Director of rugby Jim Mallinder says that he has been delighted with how Manoa has adapted to the professional game and that there is more to come in the next three years.
"Samu has adjusted to the professional game well and is starting to make his mark in Aviva Premiership Rugby," he said. "He is a natural athlete and has all the skills and physical attributes to be a good player. But he is just at the start of his professional career and there is more to come as he gets more game time and experience. So we are very happy that he has decided to sign a new three-year deal with us and look forward to seeing his development as a Saints player."