Reports in the Portuguese media said the 47-year-old was helping a colleague park a coach when they were knocked down.
It is understood the incident happened at terminal two at around 1pm.
Portuguese police confirmed the person was pronounced dead at the scene.
A spokesman for Ryanair told The Sun the employee worked for handling agent Groundlink.
"This afternoon, a Groundlink ramp agent suffered a fatal injury when parking an empty bus in a designated parking area at Lisbon Airport,” they said.
"Ryanair, through its handling agent Groundlink, expressed its sincere condolences to the family and friends of this Groundlink employee."
A spokesperson for ANA, the Portuguese airport authority, added: "An accident happened at the airport which resulted in a person’s death that we sincerely regret.
"The accident occurred in a technical area operated by Groundlink, Ryanair’s handling agent.
"ANA offers its deepest condolences to the victim’s family and stands ready to cooperate with the relevant authorities."
