The Argentine side overpowered their Japanese opponents at Lumen Field under the Seattle sun, in an entertaining battle.
Under 12,000 fans attended the clash at the 69,000-capacity stadium but those who were there, many travelling from Japan and Argentina, created a vibrant atmosphere at both ends of the stadium.
"Thanks to all River fans, we knew it wasn't going to be easy to get all the way to here, it's a very long journey, there's no direct flights," said striker Facundo Colidio, who opened the scoring for River Plate.
"Nevertheless they made it, they were here and they were amazing with their support throughout the game...
"The venue was really tricky, it's not close to Argentina."
All six South American teams that have played in the expanded Club World Cup thus far have stayed undefeated.
Marcelo Gallardo's River started the stronger and took the lead when Colidio powered a header home from former Sevilla defender Marcos Acuna's cross.
The Red Diamonds played their way into the match and River goalkeeper Franco Armani was forced into an excellent save from Yusuke Matsuo, although he was offside.
New Real Madrid signing Franco Mastantuono almost created a second for River, with the 17-year-old teeing up Nacho Fernandez, but the midfielder's drive flew narrowly off target.
The Argentine, who will join Spanish giants Madrid after the tournament, was otherwise quieter than expected.
The Red Diamonds, who won the Asian Champions League in 2022 to qualify as the only Japanese team at the tournament, shot themselves in the foot at the start of the second half.
Marius Hoibraaten sent a foolish back-header towards goalkeeper Shusaku Nishikawa, without noticing Sebastian Driussi sneaking in behind.
The striker bravely nodded home past the helpless stopper but hurt himself as he fell and had to be replaced.
Matsuo pulled the Red Diamonds back into the game from the penalty spot after Acuna's ungainly mistimed barge on Takuro Kaneko, sending Armani the wrong way.
Maximiliano Meza put the game to bed for River with a header from Acuna's corner after he was left unmarked.
"Unfortunately we lost today but we're going to learn a lot and take that into the next match," said Matsuo.
Polish Urawa coach Maciej Skorza was unhappy with the defeat.
"The result is disappointing, what can I say?" he mused.
"We've done a lot of work, put in a lot of effort, and the beginning of this game was far from what we expected.
"The second half was better but the ease with which we conceded was too much."
Elsewhere in Group E Champions League runners-up Inter Milan face Mexican team Monterrey later Tuesday at the Rose Bowl in Pasadena.
- Find more details on group standings, match schedules, and competition statistics.
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