A spokesperson for the Hessian regional police in western Germany said the body was found during maintenance work.
The plane from Tehran landed in Frankfurt on Thursday morning.
Police added that they found an oxygen tank with a mask next to the body, explaining that they did not currently have more information on the identity of the man and the circumstances of the tragedy, and indicated that they had opened an investigation.
This happens in one moment in which Iran is witnessing a wave of widespread demonstrations which are violently repressed, which so far have resulted in dozens of deaths and led to the arrest of hundreds of protesters.
Protests broke out in following the death of Mahsa Amini, 22, on September 16, three days after she was arrested in Tehran on suspicion of violating Iran’s strict women’s dress code.