There are wholesale changes in the front row, where Dragons tighthead Chris Coleman returns from the injury which ruled him out of the first two pool games and loosehead Rhys Davies (Scarlets) and hooker Dewi Lake (Ospreys) make their first start of the tournament.
There is also an all-new second row, with Bridgend's Jack Pope and Scarlets' Lewis Ellis-Jones getting their first starts in France. Likewise Dragons youngster Lennon Greggains is the new man in the back row packing down at openside.
Another Dragons back rower, Ben Fry, could make his debut from the bench, while Max Williams and Max Llewellyn - called up to the squad for the injured Rio Dyer - is also among the replacements.
Eighteen-year-old centre Tiaan Thomas-Wheeler will make his Wales U20 debut alongside experienced centre Ioan Nicholas, whose fellow Scarlet, Corey Baldwin, moves from 13 to 14.
Scrum-half Harri Morgan gets his first start of the World Rugby U20 Championship, as does his Ospreys teammate Dewi Cross (wing), who made his debut on Sunday against New Zealand.
"The new boys in the team have been chomping at the bit for a game, and the way they've been training has reflected that," said head coach Geraint Lewis. "They've kept their focus and I'm looking forward to seeing what they can do. There's a combination of experience in the team, with a freshness brought by the incoming players - all of whom are hungry to do well in Perpignan."
Wales U20 v Japan U20 will be shown live on the S4C channel, website and Facebook page.
Wales U20 team to face Japan:
15 Cai Evans (Ospreys)
14 Corey Baldwin (Scarlets)
13 Ioan Nicholas (Scarlets)
12 Tiaan Thomas-Wheeler (Ospreys)
11 Dewi Cross (Ospreys)
10 Ben Jones (Cardiff Blues)
9 Harri Morgan (Ospreys)
1 Rhys Davies (Scarlets)
2 Dewi Lake (Ospreys)
3 Chris Coleman (Dragons)
4 Lewis Ellis-Jones (Scarlets)
5 Jack Pope (Bridgend)
6 Tommy Reffell (c) (Leicester Tigers)
7 Lennon Greggains (Dragons)
8 Taine Basham (Dragons)
Replacements
16 Rhys Carre (Cardiff Blues)
17 Iestyn Harris (Cardiff Blues)
18 Rhys Henry (Ospreys)
19 Max Williams (Dragons)
20 Ben Fry (Dragons)
21 Dane Blacker (Cardiff Blues)
22 Ben Thomas (Cardiff Blues)
23 Max Llewellyn (Cardiff Blues)
