Groom will move to Northampton from Super Rugby franchise the Stormers this summer.
The 26-year-old has played 59 times for Western Province in the Currie Cup and recently hit a half-century of appearances for the Stormers in Super Rugby in their win over Queensland Reds.
He will replace Fotuali’i, whose contract comes to an end at Northampton at the end of this season.
Kettering Commonwealth Games medalist Kieran receives a hero's return
Glover’s plans for Poppies will take everything into account
Chair role to be shared at Diamonds for this season
NCL: Brigstock stun Peterborough as Finedon pull clear
Poppies boss Glover still on the hunt for fresh faces
The Samoan has been linked with a move to Saints’ Premiership rivals, Bath.
On the signing of Groom, Saints boss Jim Mallinder said: “Nic is a talented player and we believe he will be a great addition to our squad next season.
“He has shown over the last few years that he has the ability to manage a game and to captain and lead a team.
“He has a lot of experience in Super Rugby, coming up against some of the world’s best players every week, and this will be invaluable as he competes for the scrum-half position with Lee Dickson and Tom Kessell.”
Groom is looking forward to sampling new experiences at the Gardens and is hoping to help Saints secure some silverware in the seasons to come.
“I’m extremely excited to be joining up with Northampton next season,” Groom said.
“It’s a top club with a proud history and personally it’s a big honour to sign with them.
“I’m hoping to continue growing as a player and as a person and to contribute to what will hopefully be a very successful period at the club.
“Leaving Cape Town and the Stormers is going to be tough and a big change, but I’m looking forward to embracing new experiences both on and off the field.
“I have many happy memories playing in Cape Town; it’s where it all started for me.
“I’ve been here my whole career and I’m so thankful for everyone who has played a role in my journey so far.
“The coaches, players as well as the passionate fans will always have a special place in my heart.”