The pair will be one of the biggest centre combinations in Wales's history.
According to the official Welsh Rugby Union website, Llewellyn stands at 6 feet 6 inches (198 centimetres) tall and weighs 107 kilograms. James is five centimetres shorter and 4kg lighter.
Tandy has made seven changes from the 43-0 defeat to South Africa in July.
Tighthead prop Archie Griffin and second row Dafydd Jenkins make their first appearances in the competition after injury.
Nicky Smith makes his first start of the championship at loosehead prop, hooker Dewi Lake captains again, and Ben Carter joins Jenkins in the second row.
In the back row, Alex Mann is named at blindside flanker, with Jac Morgan at openside flanker and Aaron Wainwright number eight.
Tomos Williams and Sam Costelow are the half-backs, with Louis Rees-Zammit and Josh Adams on the wings and Blair Murray at full-back.
"There's lots of competition for positions within the squad, which is where we want to be," said Tandy.
"We're excited to be back at home. We want to go out there on Saturday, put our mark on the match and start the next phase in our Nations Championship campaign strongly."
Japan are the first Nations Championship visitors to Cardiff this month, with New Zealand and Australia to follow in the northern hemisphere leg.
Wales began the competition in July by beating Fiji, before losing in Argentina and South Africa.
The Nations Championship culminates in a finals weekend later this month at Twickenham, where northern and southern hemisphere teams will be paired according to their respective rankings, with an overall winner crowned.
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.
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Wales team v Japan:
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 (capt), 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