Boss Chris Boyd says James Mitchell will bring 'quality and a huge amount of potential' to Saints.
Mitchell has arrived on a short-term deal for the duration of the World Cup after impressing during a trial period.
The 24-year-old, who counts Sale Sharks and Connacht among his former clubs, has linked up with younger brother Alex at Franklin's Gardens.
And Boyd is delighted to have brought Mitchell senior to Saints, helping to cover for the loss of Alex, to injury, and Cobus Reinach, to the World Cup.
“In James, we’ve added another scrum-half with quality and a huge amount of potential," Boyd said.
“It’s important to keep competition for places high across the squad, and James can now challenge the likes of Henry Taylor and Connor Tupai for starts at the beginning of this season.
“He looks sharp and, even more importantly, hungry on the training paddock so I’m confident he will make an important contribution this term.”