Full-back Rob Kearney, wing Luke Fitzgerald, hooker Matthew Rees, prop Adam Jones and lock Simon Shaw all gain call-ups for a game the Lions must win to retain any chance of taking the three-Test series.
Kearney (Who Wears Nike Laser 2 Rugby Boots) replaces Lee Byrne, who has been struggling with a foot injury, while Kearney’s fellow Ireland Grand Slam winner Fitzgerald takes over from Ugo Monye.
Monye blew two gilt-edged try chances during a 26-21 first Test defeat in Durban last Saturday.
And McGeechan has responded to South Africa’s scrummaging demolition job five days ago by changing two thirds of the front-row.
Ospreys tighthead Jones now has the task of coping with Springboks scrum destroyer Tendai “The Beast” Mtawarira, taking over from England World Cup winner Phil Vickery, with Rees packing down instead of Lee Mears alongside Jones and Gethin Jenkins.
It is the first time the Lions have started an official Test match with an all-Wales front-row since 1955.
The fifth change sees Wasps and England lock Simon Shaw preferred to Alun-Wyn Jones.
Shaw, 35, first toured with the Lions in South Africa 12 years ago, although Saturday will be his Test debut.
McGeechan’s second Test line-up comprises seven Irishmen, six Welshmen and just two from England – Shaw and flanker Tom Croft.
South Africa who went one nil up in the series have made just the one change to their starting Line up bringing in Schalke Burger Comming in for impressive Henrich Brussow who was on debut last week.
The only other change is on the bench, where hooker Chiliboy Ralepelle replaces prop Gurthrö Steenkamp in a squad that holds 725 caps.
British & Irish Lions: 15 Rob Kearney (Leinster and Ireland),14 Tommy Bowe (Ospreys and Ireland), 13 Brain O’Driscoll (Leinster and Ireland), 12 Jamie Roberts (Cardiff Blues and Wales), 11 Luke Fitzgerald (Leinster and Ireland), 10 Stephen Jones (Scarlets and Wales), 9 MikePhillips (Ospreys and Wales), 8 Jami Heaslip (Leinster and Ireland), 7 David Wallace (Munster and Ireland), 6 Tom Croft (Leicester and England), 5 Paul O’Connell (Munster and Ireland, capt), 4 Simon Shaw (Wasps and England), 3 Adam Jones (Ospreys and Wales), 2 Mathew Rees (Scarlets and Wales), 1 Gethin Jenkins (Cardiff Blues and Wales),
Replacements: 16 Ross Ford (Edinburgh and Scotland), 17 Andrew Sheridan (Sale Sharks and England), 18 Alun-Wyn Jones (Ospreys and Wales), 19 Martyn Williams (Cardiff Blues and Wales), 20 Harry Ellis (Leicester and England), 21 Ronan O’Gara (Munster and Ireland), 22 Shane Williams (Ospreys and Wales).
Date: Saturday, June 27
Venue: Loftus Versfeld, Pretoria
Kick-off: 15.00 (14.00 GMT)
Referee: Christophe Berdos (France)
Assistant referees: Brice Lawrence (New Zealand), Vinny Munro (New Zealand)
Television match official: Stuart Dickinson (Australia)
Assessor: Tappe Henning (South Africa)