Andrew Brunson, who is currently on trial over alleged links to a failed coup in the country almost two years ago, was described by Donald Trump as a "fine gentleman".
Posting on Twitter, he said: "Pastor Andrew Brunson, a fine gentleman and Christian leader in the United States, is on trial and being persecuted in Turkey for no reason.
"They call him a spy, but I am more a spy than he is.
"Hopefully he will be allowed to come home to his beautiful family where he belongs!"
The 50-year-old from North Carolina is accused of connections to two groups which Ankara blames for an attempted overthrow of the government in July 2016.
Lawyers supporting Mr Brunson, who had spent years leading a small Protestant church in the city of Izmir, say he has been targeted specifically for his Christian faith.
Pastor Andrew Brunson, a fine gentleman and Christian leader in the United States, is on trial and being persecuted in Turkey for no reason. They call him a Spy, but I am more a Spy than he is. Hopefully he will be allowed to come home to his beautiful family where he belongs!— Donald J. Trump (@realDonaldTrump) April 18, 2018
He appeared to court on Monday to deny the charges against him before being remanded in custody for three weeks until his next court appearance before a court in Izmir.
Last year, Turkey's President Recep Tayyip Erdogan suggested Mr Brunson's fate could be linked to a his fate could be linked to that of a Muslim cleric Turkey wants extradited from the US.
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