The 27-year-old -- capped 10 times since making his Test debut in 2014 and scored the first of his two Test tries in November that year against England at Twickenham -- said he hoped to end his time in Natal on a high in the Super Rugby Championship.
"I'm delighted and excited to be joining Northampton," said Reinach, who will join the Saints ahead of the 2017/18 campaign.
"It is a club with a long tradition and strong values.
"Saints have some outstanding players in their squad and I knew after speaking with the coaches that it was a club that I wanted to play for.
"I want to thank everyone at the Sharks for all they've done for me in my career, and I'm 100 percent committed to them until the end of the season.
"I'm looking forward to the Super Rugby campaign and hopefully finishing on a high note with the Sharks and regaining my place in the Springboks' squad."
Saints director of rugby Jim Mallinder said Reinach, whose father Jaco played for the Springboks on the wing, had been a target of theirs for a while.
"Cobus is an excellent player and we're looking forward to having him at Franklin's Gardens next season," said Mallinder, who earlier this month secured Australian international centre Rob Horne for next season as well.
"We have been watching him for some time and he has proven time and again what a quality player he is.
"His set piece skills are excellent and he is a very opportunistic scrum half.
"We need quality players in each position to be able to compete at the highest level."
Saints are having a disappointing season by their high standards and are presently sixth in the Premiership, 21 points adrift of leaders Wasps.
Eyewitness? Submit your stories now via social or:
Email: eyewitness@pulse.com.gh
