A dead body was found in the undercarriage of a Lufthansa plane that arrived in Frankfurt from Iran, police said.
The man's corpse was found during maintenance work, a spokesman for the police in the state of Hesse told AFP.
The plane had landed in Frankfurt early on Thursday morning and had flown in from the Iranian capital Tehran.
An oxygen canister and a mask were found next to the body, the police said. They added that they have launched an investigation into the death.
Nationwide protests erupted in Iran following the September 16 death of Mahsa Amini, 22, after her arrest by the morality police in Tehran for allegedly violating Iran's strict dress code for women.
Her family has since been put under house arrest. Dozens of people have been killed and hundreds more detained in the unrest.