Costelow will partner Tomos Williams at halfback, with Max Llewellyn and Eddie James forming a powerful centre pairing.
Experienced Josh Adams and Louis Rees-Zammit are the two wings, with Blair Murray selected at fullback.
Dewi Lake will captain the side from hooker, with props Nicky Smith and Archie Griffin alongside him in the front row, and Dafydd Jenkins and Ben Carter the lock pairing.
Aaron Wainwright packs down at number eight with Alex Mann and Jac Morgan the two loose-forwards.
Wales, who have won two of their last four tests, also face New Zealand on November 14 and Australia a week later ahead of the Nations Championship finals weekend.
Nations Championship 2026
The inaugural Nations Championship will see six northern hemisphere and six southern hemisphere nations battle it out across six rounds in the July and November international windows before assembling in Twickenham for one unforgettable finals weekend on November 27-29.
Nations Championship fixtures | Nations Championship previews | Nations Championship standings | Nations Championship news | Everything you need to know about the Nations Championship | Where to watch the Nations Championship
Wales team:
Starters: 15-Blair Murray, 14-Louis Rees-Zammit, 13-Eddie James, 12-Max Llewellyn, 11-Josh Adams, 10-Sam Costelow, 9-Tomos Williams, 8-Aaron Wainwright, 7-Jac Morgan, 6-Alex Mann, 5-Ben Carter, 4-Dafydd Jenkins, 3-Archie Griffin, 2-Dewi Lake (captain), 1-Nicky Smith
Replacements: 16-Ryan Elias, 17-Rhys Carre, 18-Tomas Francis, 19-Teddy Williams, 20-James Botham, 21-Reuben Morgan-Williams, 22-Dan Edwards, 23-Louie Hennessey
Japan team:
Starters: 15-Kazuma Ueda, 14-Kippei Ishida, 13-Dylan Riley, 12-Samisoni Tua, 11-Inoke Burua, 10-Riku Kitahara, 9-Naoto Saito (co-captain), 8-Jack Cornelsen, 7-Kanji Shimokawa, 6-Ben Gunter, 5-Warner Dearns (co-captain), 4-Harry Hockings, 3-Shuhei Takeuchi, 2-Mamoru Harada, 1-Yota Kamimori
Replacements: 16-Kenji Sato, 17-Sojiro Otsuka, 18-Opeti Helu, 19-Michael Stolberg, 20-Tiennan Costley, 21-Kenta Fukuda, 22-Shinya Komura, 23-Jisu Lee