Two Polish soldiers were killed and four others injured on Tuesday when a bomb exploded as they were attempting to defuse it, Defence Minister Mariusz Blaszczak said.
The bomb dated back to World War II, local media reported.
The explosion occurred in the forest near the town of Kuznia Raciborska, the military said.
Two of those injured were flown to hospital in serious condition.
The troops were part of a parachute regiment stationed in the region.
According to police, they were tasked with dealing with old artillery shells discovered days earlier by people walking in the forest.
World War II bombs are still often found in Poland which was invaded by Nazi Germany.
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