The note, signed on Friday by 43 Republicans and 23 Democrats in the upper chamber of the US Congress, describes Andrew Brunson as a "law-abiding American citizen".
It brands accusations brought against the 50-year-old from North Carolina as an "absurd collection of anonymous accusations, flights of fantasy, and random character assassination".
The signatories state they endorse cooperation between American and Turkish officials concerning investigations into individuals thought to be involved in a violent bid to overthrow the Turkish government in July 2016.
They continued: "However, we are deeply disturbed that the Turkish government has gone beyond legitimate action against the coup plotters to undermine Turkey's own rule of law and democratic traditions.
"Under the State of Emergency, tens of thousands of Turkey's own citizens as well as some citizens of the United States and other countries have been arrested, dismissed from employment or otherwise seen their lives destroyed on vague charges and dubious evidence."
Brunson, who has lived in Turkey for more than 20 years and had been leading a Protestant church in the Mediterranean city of Izmir, faces up to 35 years in prison if found guilty of terrorism and espionage charges.
He was first detained in October 2016 and later accused of supporting two groups which the Turkish government blames for the attempted coup.
Earlier this week, US President Donald Trump expressed support for Mr Brunson when he tweeted: "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."
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