"The body of a man with a backpack and a Nigerian passport was found in a forest near Olkhovka," the police said in a statement.
The body was found by soldiers about six miles away from the Polish-Belarusian border, Tomasz Krupa, a spokesman for the regional police department, said. The identity and cause of death were being established.
Since the beginning of the year, Polish border guards have recorded about 40,000 attempts to illegally cross the border from Belarus, which is ten times more than in 2020.
Several thousand migrants camp on the Polish-Belarusian border in the hope of forcing their way into the European Union.
The Polish authorities strengthened border security and thwarted attempts by illegal migrants to enter the country, using teargas and barbed wire. Warsaw blames Minsk for the current migration crisis. Belarus has so far rejected the allegations.