Morgan, 20, is the only change in the backline and partners Jamie Roberts in midfield. Alex Cuthbert will make his 40th Wales appearance in a back three alongside George North and full-back Gareth Anscombe.
Dan Biggar and Gareth Davies continue their half-back partnership with Biggar looking to add to his RWC tally of 42 points.
Jenkins comes back into the starting XV to feature in his 18th RWC match and he packs down alongside Scott Baldwin and Samson Lee.
Luke Charteris and Alun Wyn Jones continue in the second row with the later making his 100th International appearance (94th for Wales, plus six British and Irish Lions appearances).
Dan Lydiate returns to the back row alongside Captain Sam Warburton and Taulupe Faletau.
“After coming through an incredibly tough Pool, we are excited to be in the quarter-finals and returning to Twickenham,” said Wales Head Coach Warren Gatland.
“The physicality and intensity of the matches so far will stand us in good stead for what is going to be a huge battle against the Springboks.
“They have grown game on game in this tournament and we know the challenge that faces us from a talented South African side.”
Ken Owens, Paul James and Tomas Francis are named as the front-row replacements with Bradley Davies and Justin Tipuric completing the forward contingent. Lloyd Williams, Rhys Priestland and James Hook are named as the replacement backs.
WALES TEAM TO PLAY SOUTH AFRICA:
Gareth Anscombe (Cardiff Blues), Alex Cuthbert (Cardiff Blues), Tyler Morgan (Newport Gwent Dragons), Jamie Roberts (Harlequins), George North (Northampton Saints), Dan Biggar (Ospreys), Gareth Davies (Scarlets), Gethin Jenkins (Cardiff Blues), Scott Baldwin (Ospreys), Samson Lee (Scarlets), Luke Charteris (Racing 92), Alun Wyn Jones (Ospreys), Dan Lydiate (Ospreys), Sam Warburton (CAPT) (Cardiff Blues), Taulupe Faletau (Newport Gwent Dragons).
Replacements: Ken Owens (Scarlets), Paul James (Ospreys), Tomas Francis (Exeter Chiefs), Bradley Davies (Wasps), Justin Tipuric (Ospreys), Lloyd Williams (Cardiff Blues), Rhys Priestland (Bath Rugby), James Hook (Gloucester Rugby).
