The 39-year-old Iranian-American has been in custody for more than a year and his trial has been held behind closed doors. His family and employer have denounced the proceedings as a sham.
The official IRNA news agency reported that the latest hearing was under way but gave no details about Rezaian's court appearance.
His lawyer has said she was told that today's trial hearing -- the fourth in the case -- would be the last but she could not "be 100 per cent sure."
"The court decides which hearing will be the last one," said Gholam Hossein Esmaili, chief of Tehran's justice department.
"Until then one cannot judge about it."
Rezaian's trial, on charges of "espionage, collaboration with hostile governments, gathering classified information and disseminating propaganda against the Islamic republic," started in May.
The case has been heard by a Revolutionary Court, which usually presides over political cases or those related to national security.
Rezaian, the Post's Tehran correspondent, was arrested with his wife Yeganeh Salehi, also a journalist, at their home in Tehran on July 22, 2014.
Rezaian's wife has not worked since and she is barred from discussing the case.
Washington Post executive editor Martin Baron on Saturday criticised the trial and the conditions of Rezaian's incarceration.
"Iran has behaved unconscionably throughout this travesty of a case," he said.
"It has imprisoned an innocent journalist for more than a year and subjected him to physical mistreatment and psychological abuse.
"The secret court proceedings that began on May 25 have been a farce.
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