The 27-year-old, who plays for the Lions in Super Rugby and Golden Lions in the Currie Cup, will join Connacht next season.
Boshoff was capped for the Springboks against Scotland in June 2014.
"The rugby Connacht are playing is really exciting and I believe that I can fit in and add something - I can't wait to get stuck in," said Boshoff.
He added: "It's clear that there is a huge amount of passion and ambition there in terms of where they want to go."
Boshoff made his Super Rugby debut in a man-of-the-match performance in the opening round of the 2014 season, scoring all the Lions' points in a 21-20 win over the Cheetahs.
He has scored 214 points in 29 Super Rugby appearances.
Boshoff has played for the Golden Lions since 2013, scoring 465 points in 25 games for the current champions.
Irish side Connacht have signed South African fly-half Marnitz Boshoff on a two-year deal.