Zimbabwean-born Mujati made his Test debut last year and has since tallied 12 international appearances for the Boks but he has opted to play his rugby outside of South Africa by signing a two-year deal at Franklin's Gardens.
With his Super 14 side the Stormers no longer in contention for a semi-final berth, Mujati's only remaining domestic commitments in South Africa will come in the shape of a Currie Cup campaign with Western Province.
Thos commitments could last as long as October, with Mujati set to move to England once his team's involvement in the competition has come to an end.
"I'm excited about the prospect of playing in England," said Mujati, who could face future team-mate Euan Murray for either the Boks or Western Province during the Lions' 10-match tour of the Republic.
"I played there with the Springboks last year and I'm sure I will like living and playing my rugby there, especially at a club like Northampton, which has done well this year.
"I've watched the Premiership on TV and it is a strong league. It will be a new experience for me and a different challenge in a new setting."
Mujati effectively replaces twice Lions tourist Tom Smith at the Saints after the 37-year-old Scotsman decided to retire at the end of this season. Smith won six successive Test caps for the Lions and was a hero in Northampton.
With Mujati's best years seemingly ahead of him, the club's director of rugby Jim Mallinder is hoping the Springbok star can have a similar impact in the East Midlands.
"Brian is one of the rising stars of South African rugby and I am delighted that he is coming to the Saints," said Mallinder.
"Although he is still relatively young for a prop he has amassed a great deal of experience in the Super 14 and with the Springboks, and he can play on both sides of the scrum.
"You cannot have too much strength in depth in the front row and Brian will add to our resources in that area considerably.
"Having quality players increases the pressure on everyone to perform, which can only be good for the Saints squad as we look to improve further."