The Iraqi man, who was part of a group of migrants detained 500 metres from the Belarusian border, is believed to have died of a heart attack and passed away despite the efforts of a border patrol and ambulance paramedics to save him.
Another member of the group has been hospitalised following a positive Covid-19 test, the Border Guard added.
The death brings the number of migrants who have now died on the Polish side of the border with Belarus to four.
Reacting to the latest death, Mateusz Morawiecki, the prime minister, said Poland would try to help all irregular migrants who had been found crossing the border and would try to save their lives. "We want each to be treated with dignity," he said.
"We are sensitive to human suffering and where we can we will treat in our hospitals and help all those in need by ensuring them adequate resources to live," he continued.
Since September 2, a 30-day state of emergency has been in force in parts of the Podlaskie and Lubelski provinces adjacent to Poland's border with Belarus due to increased migratory pressure. Since the start of September, about 5,000 illegal border-crossing attempts have been recorded on the Polish-Belarusian frontier.
Poland and the Baltic States have accused the Belarusian government of pushing migrants across their borders in an effort to destabilise both them and the EU as part of a "hybrid war." (PAP)