"Iran has behaved unconscionably throughout this case, but never more so than with this indefensible decision by a Revolutionary Court to convict an innocent journalist of serious crimes after a proceeding that unfolded in secret, with no evidence whatsoever of any wrongdoing," the paper said shortly following the reported announcement on Iranian state television.
No sentence for Rezaian was announced and over the weekend, when an Iranian judiciary spokesperson first said a verdict had been reached, the official noted it could be appealed and "is not final," according to The Washington Post. Rezaian faced a number of charges including espionage and it is unclear of which he has been convicted.
The Post said today it is working with Rezaian's Iranian counsel "to pursue an immediate appeal."
"The contemptible end to this ‘judicial process’ leaves Iran’s senior leaders with an obligation to right this grievous wrong. Jason is a victim -- arrested without cause, held for months in isolation, without access to a lawyer, subjected to physical mistreatment and psychological abuse, and now convicted without basis," the Post said.
Hours before the guilty verdict, Jason Rezaian's brother, Ali, told ABC News he was still in disbelief at the mistreatment of Jason.
"I think it's absolutely unbelievable that he would be held for this long," Ali said. "For whatever reason, we don't know why they took him. What we do know is there's no evidence against him, and they've held him essentially in solitary confinement for the last 14 months."
Rezaian was arrested July 22, 2014 and now has been held longer than the Americans detained during the 1979 Iranian hostage crisis. Rezaian’s only contact with the outside world has been with his Iranian wife and his mother who told ABC News her son wishes he could be covering the news instead of being the news.
Rezaian's closed-door trial ended two months ago and no Iranian officials have explained the delay in announcing a verdict.
"It's unbelievable that they would treat anybody like this, let alone somebody from The Washington Post, a journalist with international visibility," Ali said.
Today U.S. State Department spokesperson said the department is aware of the news or the conviction, but had not seen “official confirmation of a verdict on specific charges or any further information.”
“Unfortunately, this is not surprising given that the process has been opaque and incomprehensible from the start,” Kirby said. “Regardless of whether there has been a conviction or not, we continue to call for the government of Iran to drop all charges against Jason and release him immediately.”
In addition to Rezaian, the Iran government is holding two other Americans in prison and is suspected of at least knowing the location of a third American who disappeared more than eight years ago in Iran.
Amir Hekmati, a former U.S. Marine, was arrested in 2011 and accused of working for the CIA. Despite the U.S. government's and Hekmati's family denails, Hekmati was sentenced to death -- a sentence that was then overturned.
“Our family is saddened over the recent news regarding Jason’s case," the Hekmati family said in a statement. "We, too, know the pain his family is experiencing, and we feel it still waiting for our own brother and son’s release."
Saeed Abedini is an American pastor arrested in Iran in 2012 while setting up churches there.
ABC News' Justin Fishel contributed to this report.