In 2010, at just 18 years and 25 days old Prydie turned out on the wing for Wales in a team featuring Sam Warburton and Gethin Jenkins and helped the side to a 33-10 win at the Millennium Stadium.
Prydie has so far gained four senior caps and will line up at full-back in Swansea as part of a back three which includes Cardiff Blues wing Harry Robinson - who has been called up by Wales caretaker head coach Rob Howley for his summer training squad.
Under-20s Head coach Danny Wilson has chosen Blues lock Cory Hill to captain the side, which mustered two wins against Scotland and Italy during the RBS 6 Nations this year.
"This match against Japan is a very important part of our Junior World Championship preparation," said Wilson.
"It is the first real chance to see some of the players in action who have had little or no game time for us this season, and after a very intense and competitive series of training sessions, that will be exciting to see.
"The game will give us a chance to see almost all of our squad, along with some of our reserve players and give players a chance to stake a claim for places in the starting line-up against Fiji, our opening JWC Pool game.
"We are working with the Ospreys to manage their players who obviously have a RaboDirect Pro12 final to prepare for but everyone should have some game time.
"Japan narrowly lost to a Welsh Academicals side in a very high scoring game on the weekend and they were very competitive against Samoa in the Junior World Trophy final last season so they will be no walk-over but our main focus is on fine-tuning our own game."
15 Tom Prydie
14 Harry Robinson
13 Jack Dixon
12 Owen Williams
11 Luke Morgan
10 Sam Davies
9 Jonathan Evans
1 Rob Evans
2 Kirby Myhill
3 Samson Lee
4 Cory Hill (c)
5 Matthew Screech
6 Luke Hamilton
7 Daniel Thomas
8 Dan Baker