Winger Rafael Leão opened the scoring for the home team with a header in the 59th minute at Dragon Stadium in Porto.
Captain Cristiano Ronaldo doubled Portugal's lead with a penalty in the 72nd minute, followed by Bruno Fernandes making it 3-0 eight minutes later.
Pedro Neto capitalised on a defence lapse in the 83rd minute to extend the score to 4-0, and Ronaldo added a fifth with a stunning bicycle kick in the 87th minute.
Poland managed a late consolation goal through midfielder Dominik Marczuk, who found the net in the 88th minute.
The Polish side was without their star striker Robert Lewandowski, sidelined by injury.
Poland's goalkeeper Marcin Bułka conceded five goals on Friday night. Photo: PAP/Leszek Szymański
Captained by Inter Milan midfielder Piotr Zieliński, the Poles failed to deliver a strong performance against their higher-ranked opponents in an away fixture.
Poland are ranked 31st worldwide by football's governing body FIFA, while Portugal are No. 7 in the FIFA table.
Poland opened their Nations League campaign by defeating Scotland 3-2 in Glasgow on September 5.
In their second game in Group 1 of the top League A, the Poles lost 0-1 to Croatia in Osijek on September 8, followed by a 1-3 loss to Portugal at Warsaw's National Stadium on October 12.
On October 15, Poland played out a 3-3 draw with Croatia at home in their fourth match of the 2024/2025 UEFA Nations League.
After five rounds of fixtures, Poland sit third in their Nations League Group A1 with four points, ahead of last-place Scotland on goal difference. Group leaders Portugal have 12 points, while Croatia are in second with seven.
Poland will take on Scotland in Warsaw on Monday in their final Nations League group-stage match, with no chance of advancing to the competition's quarterfinals.
The Nations League is the European football governing body UEFA’s newest national-team competition that is now in its fourth edition.
Portugal won the first edition of the Nations League in 2019, while France won the 2020/2021 UEFA Nations League, and Spain triumphed in the 2022/2023 edition.
Poland manager Michał Probierz has said that this season's Nations League offers a valuable opportunity to assess players and prepare them for 2026 World Cup qualifiers.
Poland boss Michał Probierz. Photo: PAP/Leszek Szymański
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Source: IAR, PAP, TVP Sport