The 32-year old, who has won 12 caps for his country, has been signed to provide injury cover.
Ospreys tight heads Dmitri Arhip and Ma'afu Fia are currently sidelined and Rhodri Jones is in the Wales squad.
"Brian has proven quality at the top level and will add so much experience to the squad, particularly over the coming weeks," coach Steve Tandy said.
Mujati has played in Super Rugby with Lions and Stormers, in Top 14 in France with Racing Metro, and in England with Northampton Saints and Sale.
At Northampton Mujati was a losing Heineken Cup and Premiership finalist, and an Anglo-Welsh Cup winner.
Ospreys Head Coach, Steve Tandy, said he is expecting Mujati to make a big impact.
"This is an important signing for us given the injury situation with Dmitri and Ma'afu. Our performances this season have put us in a really strong position but we still have eight PRO12 games to go, and a Challenge Cup quarter-final in April," he said.
"In Rhodri Jones and Daniel Suter we have two young men who will be heavily involved but we can't go through such a big run of games with only two fit pro tight heads on our books, particularly with Rhodri still a part of the Six Nations squad and potentially away on Wales duty. It's a critical position and we felt that we had to act now."

It is the second time the hooker has skippered the Lions during the tour, having led them to a win over the Chiefs.
Coach Warren Gatland made the decision after the Lions lost the opening Test against New Zealand 30-15 on Saturday.
The second Test against the All Blacks is on Saturday, 1 July.
"We are obviously hugely disappointed with the result but it is important to regroup," said Gatland.
"We know there are a lot of players motivated to go out and get a result for the whole group.
"A few players put their hands up for selection from the Chiefs game and played their way into the Test team so they know that there is another opportunity for them on Tuesday against the Super Rugby champions."
Wing George North and centre Jonathan Joseph will both start against the Hurricanes, while Jack Nowell is at full-back.
Lock George Kruis is on the bench having played the whole 80 minutes of the first Test against New Zealand.
The All Blacks have released Julian Savea, Jordie Barrett and Ngani Laumape back to the Hurricanes for the game, while Nehe Milner-Skudder also starts.
British and Irish Lions: Jack Nowell (England), Tommy Seymour (Scotland), Jonathan Joseph (England), Robbie Henshaw (Ireland), George North (Wales), Dan Biggar (Wales), Greig Laidlaw (Scotland), Joe Marler (England), Rory Best (Ireland, captain), Dan Cole (England), Iain Henderson (Ireland), Courtney Lawes (England), James Haskell (England), Justin Tipuric (Wales), CJ Stander (Ireland).
Replacements: Kristian Dacey (Wales), Alan Dell (Scotland), Tomas Francis (Wales), Cory Hill (Wales), George Kruis (England), Gareth Davies (Wales), Finn Russell (Scotland), Jared Payne (Ireland).
Hurricanes: Jordie Barrett, Nehe Milner-Skudder, Vince Aso, Ngani Laumape, Julian Savea, Otere Black, Te Toiroa-Tahuriorangi, Ben May, Ricky Ricitelli, Jeffrey To'omaga-Allen, Mark Abbott, Sam Lousi, Vaea Fifita, Callum Gibbins, Brad Shields (captain).
Replacements: Leni Apisai, Chris Eves, Mike Kainga, James Blackwell, Reed Prinsep, Kemara Hauiti-Parapara, Wes Goosen, Cory Jane.
Ospreys have signed Sale Sharks' former South Africa tight head prop Brian Mujati until the end of the season.
[NEXT_CONCEPT]
Rory Best will captain the British and Irish Lions for his side's next game against the Hurricanes in Wellington on Tuesday.