The victory marked Poland’s second over Japan in this year’s tournament, after a 3-1 win in Xi'an, China in June, Polish state news agency PAP reported.
Poland will face Brazil on Saturday for a spot in the gold medal match.
Slovenia, who overcame reigning champions France 3-1 earlier on Thursday, will face Italy in the other semifinal.
Poland are aiming for their sixth consecutive Nations League medal after they won gold in 2023, silver in 2021, and bronze medals in 2019, 2022 and 2024.
Marcin Komenda, a key player for Poland, told reporters that although the team did not drop a set against Japan, it had to fight hard to push its way into the medal round.
"It’s good that we won in three sets because I think it might help us recover a bit faster before the semifinal clash," he said.
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Source: IAR, PAP