Markus Loening said Wednesday the German government is extremely concerned about Yusuf Naderkhani's fate amid reports of his imminent execution.
Loening urged Tehran to "release Naderkhani, lift his death sentence and grant him a fair trial."
He says an execution also breaches international obligations guaranteeing religious freedom.
Naderkhani, who is in his early 30s, converted to Christianity when he was 19 and later became a pastor in the Iranian city of Rasht. He was convicted of apostasy in 2010.
The U.S. government called for his release last year and Germany summoned the Iranian ambassador to protest Naderkhani's continued imprisonment.