The win means England’s semi-final opponents will be France, a side who they beat 19-8 in this year’s Six Nations.
Tries from Wasps duo Elliot Daly and Christian Wade, and Newcastle Falcons No8 Alex Gray helped England to the success, while Leicester Tigers fly-half George Ford scored 11 points with the boot.
Ford’s Leicester Tigers colleagues Ryan Bower and Matt Everard came off the bench to share in the victory.
England got off to a blistering start with Ford scoring a long-range penalty before Daly touched down in the corner.
South Africa came back into the game with a try from a lineout but a Ford penalty gave England an 11-5 lead minutes later.
Johan Goosen levelled the scores with two penalties before the break.
England started the second half just as they had done the first, captain Gray rounding off an excellent cross-field move to score in the corner.
After Goosen landed another penalty, Ford’s next kick penalty momentarily reclaimed England’s five-point lead, but a South Africa three-pointer made the score 19-17 with 15 minutes to play.
Wade then scored his fourth try of the competition, displaying great pace on the left, and Ford’s conversion put England nine points in front.
As the clock ticked past 80, Saracens prop Mako Vunipola was sin-binned at a ruck and Goosen scored the penalty to make the score 26-20.
England U20: 15 Ben Ransom (Saracens), 14 Andy Short (Worcester Warriors), 13 Elliot Daly (London Wasps), 12 Owen Farrell (Saracens), 11 Marland Yarde (London Irish), 10 George Ford (Leicester Tigers), 9 Chris Cook (Bath Rugby); 1 Mako Vunipola (Saracens), 2 Mikey Haywood (Northampton Saints), 3 Henry Thomas (Sale Sharks), 4 Joe Launchbury (London Wasps), 5 Charlie Matthews (Harlequins), 6 Sam Jones (London Wasps), 7 Matt Kvesic (Worcester Warriors), 8 Alex Gray (c) (Newcastle Falcons). Replacements: 16 Koree Britton (Gloucester) on for Haywood 79 min, 17 Ryan Bower (Leicester Tigers) on for Thomas 65 min, 18 Sam Twomey (Harlequins), 19 Matt Everard (Leicester Tigers) on for Jones 76 min, 20 Dan Robson (Gloucester), 21 Guy Armitage (London Irish) on for Daly 45 min, 22 Christian Wade (London Wasps) on for Yarde 57 min
