Jason Rezaian was arrested in July 2014 at his home in Tehran; he had been working there as a correspondent for the American newspaper for two years.
Iranian-American journalist Jason Rezaian spent his 500th day in Iran's Evin prison on Wednesday, as his employer again demanded his freedom.
The Washington Post reporter has been convicted of espionage by a secretive Iranian court and is serving an unspecified prison sentence in the notorious jail.
The Post has denounced his trial as a sham, and his case has been taken up by the United Nations Working Group on Arbitrary Detention, but to no avail.
"Jason has done nothing wrong," said The Post's executive editor, Martin Baron. "Iran has never even bothered to produce any evidence against him."
"Recent announcements, without details, of his supposed conviction and sentencing, have only added new layers of cruelty," he said.
"Iran's senior leaders have said for months that they would like Jason's case to be resolved," Baron added. "The fact that Jason is still behind bars calls their credibility and commitment into question."
To mark the 500th day of Rezaian's imprisonment, The Post submitted a supplemental report to the UN working group following the case.
It alleges 39-year-old Rezaian's health is deteriorating because of the harsh conditions in Evin, and it details what The Post sees as "egregious violations of basic due process" during his trial.
"He suffers from blood pressure complications and other physical and mental medical conditions that remain untreated or poorly treated," the report says.
"He has been subjected to further interrogations, psychological abuse and physical mistreatment.
"He is deprived of normal human interactions and forced to wear a hood when he is escorted around the prison by guards or interrogators."
Rezaian was arrested in July 2014 at his home in Tehran; he had been working there as a correspondent for the American newspaper for two years.
His wife, Yeganeh Salehi, was also arrested but she was later released on bail.
Since May, Rezaian has appeared four times behind closed doors before Tehran's Revolutionary Court, which presides over politically charged cases.
Foreign Minister Mohammad Javad Zarif said in October that Iran wanted to resolve the case "from a humanitarian point of view."
That statement fed speculation that Tehran would consider a prisoner swap for Iranians held in the United States for trying to break US economic sanctions.
The United States has repeatedly called for Rezaian's release.