Squad captain Sam Warburton and fellow Blues back row forward Dan Franks both miss out with shoulder injuries picked up in the championship semi-final against New Zealand on Wednesday night, while Dragons wing Jimmy Norris suffered a knee injury.
Eighteen year old Blues U20 flanker Jake Thomas has been called into the squad and will take up a place on the bench.
Wales U20 Head Coach Patrick Horgan said, "South Africa are a very efficient side and it will take a very good performance to beat them. Having said that, if we can pull out all the stops and get a win on Sunday, not only will we finish third in the world, but the only two teams to have beaten us this season will be the Junior World Championship finalists England and New Zealand."
Horgan believes this is a great chance for Wales U20 players to impress onlookers.
"While a number of our players will still be eligible for next season's U20 squad, for others, this will be their last international age grade match. It's a fantastic opportunity for the players to play against another southern hemisphere side and stake a claim for places in Premiership and regional sides next season.
"The tournament has been played at a very high intensity and there are a lot of tired bodies in the squad but everyone is very upbeat and keen to perform well on Sunday."
Finals day tickets are on sale at just £2 for children and £8 for adults. Fans are advised to purchase tickets for the Liberty Stadium finals day well in advance of kick-off. The Liberty Stadium Box Office is open Friday until 6pm, Saturday between 10am to 4pm and from 12 noon on Sunday. Fans can also pre-book by calling 08700 400 004.
Wales U20: Jason Tovey (Dragons); Leigh Halfpenny (Blues), Jonathan Davies (Scarlets), Rhys Williams (Scarlets), Nathan Rees (Blues); Gareth Owen (Ospreys), Gareth Williams (Ospreys); Ryan Bevington (Ospreys), Ryan Prosser (Bristol), Scott Andrews (Blues), Haydn Pugh (Scarlets), Ashley Sweet (Dragons), Josh Turnbull (capt, Scarlets), Justin Tipuric (Ospreys), Lloyd Phillips (Scarlets)
Replacements: Ben Roberts (Dragons), Sam Hobbs (Blues), Pat Palmer (Blues), Jake Thomas (Blues), Rhys Webb (Ospreys), Dan Biggar (Ospreys), Daniel Evans (Scarlets)