"Pastor Andrew Brunson, a fine gentleman and Christian leader in the United States, is on trial and being persecuted in Turkey for no reason", Mr Trump tweeted on Tuesday.
Brunson, who's accused of espionage and assisting terror bands, needs to really be allowed to "allowed to come home to his family at which he belongs", Trump explained.
Andrew Brunson, a Christian pastor from North Carolina, U.S. who has been in jail in Turkey since December 2016, is seen in this undated picture taken in Izmir, Turkey.
He is accused of aiding the Kurdistan Workers' Party, which opposes Erdogan and the ruling party, and of having a connection to Fethullah Gulen, an Islamist preacher who Turkey claims orchestrated the coup attempt. "I want truth to come out".
US officials have been working to release Brunson since he was arrested in 2016, following a failed coup against Turkish President Recep Tayyip Erdogan, resulting in more than 50,000 arrests.
After a grueling first day - nearly 13 hours where "secret witnesses" testified via video link with their faces and voices obscured to the point of being almost incomprehensible - the Turkish judge ordered Pastor Andrew to be returned to prison.
"I've never done something against Turkey", Brunson said, rejecting the charge of his involvement in the Gulen Movement. Sam Brownback, the U.S. Ambassador at Large for Religious Freedom. I'm not in favor of separation.
Brunson reacted emotionally as the ruling was read out, telling his wife Norine in English: "I am going insane". I reject all the accusations in the indictment. "Of course I have hope; I want to get a call saying my father is coming home".
We're directly involved in directing his legal defense in Turkey.
"I've never done something against Turkey". On the contrary, I love Turkey.
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"That relationship is going to have difficulty moving forward as long as Andrew Brunson is incarcerated", Brownback told reporters at the trial, according to Reuters.
The case has raised tensions - further - between the USA and Turkey, which in recent years has been regressing toward a Shariah-compliant theocracy.
"I am a Christian warrior", Brunson mentioned throughout the hearing.
Gulen denies any role in the failed coup and says his Hizmet (Service) movement promotes a peaceful form of Islam.
Last May, at a face-to-face meeting with Erdogan in Washington, Trump called for Brunson's release, the White House said at the time.
In February, NASA scientist Serkan Golge, a dual national, was jailed for 7 and a half years for being a member of Gulen's movement in a conviction denounced by Washington.
Numbering just several thousand, the Protestant community in overwhelmingly Sunni Muslim Turkey largely comprises converts from Islam, expatriates and refugees.
The US State Department meanwhile said it had seen "no credible evidence" Brunson was guilty of a crime.
The two countries are at odds over USA support for the Kurdish militia in northern Syria, as Turkey considers the militia a terrorist organization and launched a military campaign against it in late January, raising fears that Turkish and American troops could come face-to-face in a potential conflict in war-torn Syria.