"We had been waiting around half-an-hour to get out of the plane," Fernando del Valle, another passenger, told AFP of the incident on Monday evening.
"The man calmly went to the emergency exit, pulled the mechanism to open the door, looked out, came back to get his backpack and went out on the wing."
Del Valle filmed the incident and posted it on his Facebook page. His footage reveals a man walking on the wing, putting his backpack down and sitting next to the bag.
"The captain was stunned... he asked who had got out on the wing," said del Valle, a Spaniard who was coming back from London on the flight which left an hour late but arrived around 20 minutes after the planned time late on Monday.
The man, whose age and identity have not been revealed, was immediately arrested, Ryanair said in a statement.
A spokesman for Spain's Guardia Civil police force, meanwhile, said the captain had asked them to intervene, adding they had reported the passenger to air safety authorities.