Former Wallaby Harrison, who has been left out of the Ulster squad for their recent run of games, will join Wales international Brent Cockbain in the second row.
Jason Robinson, who injured his shoulder in England's World Cup final defeat to the Springboks, is named at full-back but his fitness is being monitored closely by Ireland coach Eddie O'Sullivan, who is in charge of the Barbarians for this game.
O'Sullivan said: "Jason has made a miraculous recovery after working incredibly hard to be fit for the game. He has been speaking to the Barbarians about playing in this match for a long time.
"He had a run out yesterday and was fine, but we are assessing him closely."
O'Sullivan's selection includes five All Blacks - flying winger Joe Rokocoko being one of them - while Australian star Matt Giteau is taking on the out-half duties.
The game is being televised live on Sky Sports 2.
BARBARIANS (v South Africa): Jason Robinson (England); Joe Rokocoko (Blues/New Zealand), Conrad Smith (Hurricanes/New Zealand), Ma'a Nonu (Hurricanes/New Zealand), Isoa Neivua (Warriors/Fiji); Matt Giteau (Western Force/Australia), Justin Marshall (Ospreys/New Zealand); Federico Pucciarello (Munster/Italy), Mark Regan (Bristol/England) (capt), Salesi Ma'afu (Brumbies), Brent Cockbain (Wales), Justin Harrison (Ulster/Australia), Rocky Elsom (Waratahs/Australia), Martyn Williams (Cardiff/Wales), Jerry Collins (Hurricanes/New Zealand).
Replacements: Schalk Brits (Stormers), JD Moller (Stormers), Troy Flavell (Blues/New Zealand), Michael Owen (Dragons/Wales), Tom Shanklin (Cardiff/Wales), Peter Grant (Stormers), Ben Cohen (England).