Washington has called for the release of Rezaian, who went on trial behind closed doors on Tuesday, with National Security Council spokeswoman Bernadette Meehan condemning a "complete lack of transparency" in the case.
Rezaian, 39, is accused of "espionage, collaboration with hostile governments, gathering classified information and disseminating propaganda against the Islamic republic", according to his lawyer.
Foreign affairs spokeswoman Marzieh Afkham said it was up to a judge to decide the journalist's fate.
"There is no room for premature judgement and speculation."
Rezaian, an Iranian-American, has been held since July last year in a politically sensitive case that has unfolded while Iran and world powers conduct nuclear talks.
Tehran does not recognise dual nationality, and says the case is a purely Iranian matter.
Rezaian's wife Yeganeh Salehi, a fellow journalist, also appeared before the court on Tuesday along with a female press photographer, Iran's official IRNA news agency reported.
The two women were detained at the same time as Rezaian but have been granted bail.
"As journalists Jason and I spoke often about the Middle East. But at no point did we do anything aside from professional reporting," Setrakian wrote on his Twitter. "I pray for Jason, for his release, and for better days between Iran and the rest of the world."
State media have given no indication of how long the trial might last.
But it will overlap with the final stretch of negotiations between Iran and the major powers aimed at reaching a comprehensive agreement on Tehran's nuclear programme by a June 30 deadline.