The man climbed over a 3-metre-high fence topped with barbed wire at Wrocław Airport, Polish state news agency PAP reported.
Wrocław Airport spokesman Bartosz Wiśniewski said the man took advantage of an opportunity while staff were assisting another passenger and mingled with passengers heading to the aircraft.
He managed to board the plane, but cabin crew noticed that he did not have a boarding pass and was not authorised to fly. They alerted the Border Guard.
Officers from the Border Guard's crisis management team entered the aircraft and detained him.
Border Guard spokesman Capt. Paweł Biskupik said the man was legally residing in Poland. No dangerous items were found, and authorities said he posed no threat.
Wrocław Airport authorities said they would conduct a thorough review of the incident.
The flight to Palermo was delayed by two hours.
(sp/gs)
Source: PAP