It said on Twitter that the illegal border crossers hailed from countries including Yemen, Egypt and Syria and had arrived in Belarus from Russia.
A group of 21 men were detained after they crossed to the Polish side through the Świsłocz River in the east of the country, the Border Guard tweeted.
"The foreigners came to Belarus from Russia," it said. "All of them were detained."
Since the start of this year, there have been more than 6,000 attempts to cross illegally into Poland from Belarus, the border agency's data showed.
Poland last year introduced new, stricter border protection rules to stave off an influx of illegal migrants from Belarus.
At the end of last month, officials marked the completion of a 187-kilometre-long protective wall designed to keep out illegal migrants along Poland's border with the Russian ally.
Last year, the European Union, NATO and the United States accused Belarus' strongman leader Alexander Lukashenko of orchestrating a migrant crisis at the Polish-Belarusian border in retaliation for Western sanctions against his regime.
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Source: IAR, PAP