Rezaian's wife, Yeganeh Salehi, who is also a journalist, appeared in court alongside her husband and a press photographer, the official IRNA news agency reported.
In a statement Monday condemning the legal proceedings against its journalist, the Washington Post had said it expected Salehi to be tried separately.
Rezaian, 39, who has been in custody in Iran for 10 months, is accused of "espionage, collaboration with hostile governments, gathering classified information and disseminating propaganda against the Islamic republic," according to his lawyer Leila Ahsan.
She has condemned the charges saying there is "no justifiable proof" to substantiate them.
The trial is being held in Branch 15 of the Tehran revolutionary court, which usually presides over political cases or those related to national security.
Rezaian, an Iranian-American, was arrested in July last year in a politically sensitive case that has unfolded while Iran and world powers conduct nuclear talks. He has been held in Tehran's notorious Evin prison.
The United States and the Washington Post have branded the charges against him absurd and urged his release.
Tehran does not recognise dual nationality, and says the case is a purely Iranian matter.
Salehi was arrested with her husband but later released on bail. The third defendant, whose name was not disclosed, was detained on the same day.