Morawiecki made the announcement on Saturday during a visit to Mysliwiec in north-central Poland.
His comment came after several media reported that Polish farmers are concerned about Ukrainian grain being sold on the country's market.
Morawiecki said that he had instructed Poland's agriculture minister to analyse the situation carefully and "stop the process."
"The grain that is coming to Poland from Ukraine is to be sold in Africa and the Middle East to ensure the food security of that region," he said.
"Poland has its own food, its own grain, and we will protect... Polish farmers," he added.
Ukraine, the world's fourth-largest grain exporter, has not been able to export its harvest to the world due to the Russian invasion and the subsequent blockade of Black Sea ports. Before the war, many African countries relied on deliveries from Ukraine.
At a Wednesday meeting of the International Team for the Transport of Food and Agriculture Products from Ukraine, Henryk Kowalczyk, Poland's agriculture minister, said that Poland was obliged to assist Ukraine export its grain to help counteract famine in Africa and Arab countries. (PAP)