The away side's Michy Batshuayi scored the only goal of the match with a header in the 16th minute at the National Stadium in Warsaw.
Poland went into the match ranked 26th worldwide by football's governing body FIFA, while Belgium were No. 2 in the FIFA table.
After four rounds of fixtures, Poland are third in their Nations League Group A4, with four points, three less than Belgium and six less than the Netherlands.
In their previous three Nations League games, the Poles beat Wales 2-1 on home turf, lost 1-6 to the Belgians in Brussels, and tied 2-2 with the Netherlands in Rotterdam.
The Polish team next takes on the Netherlands at home on September 22.
The Nations League is the European football governing body UEFA’s newest national-team competition that is now in its third edition.
The competition is designed to enable teams to play against those that are similarly ranked and may also offer an alternative path for teams to qualify for the 2024 European championships.
Portugal won the first edition of the Nations League in 2019, while France won the 2020/2021 UEFA Nations League after beating Spain 2-1 in the final in Milan in October.
Source: TVP Sport, PAP, uefa.com