They remained publicly undisclosed until last week.
The Iranian-American will be tried soon on espionage, Tehran's chief justice said. He is accused of economic spying, the Post reported, citing Iranian state media.
The Washington Post did not mince words on the allegation. "Any charges of that sort would be absurd, the product of fertile and twisted imaginations," the paper said in a statement.
The State Department also reacted with term "absurd" after hearing of reports in Iran's press about the charges.
"If the reports are true, these charges are absurd, should be immediately dismissed and Jason should be immediately freed so that he can return to his family," the State Department official said.
An appeal from Muhammad Ali
Since officers picked up Rezaian and his wife, Yeganeh Salehi, on July 22 at their home, the Post, the State Department and Rezaian's family have protested and called for his release. Salehi was released on bail in October.
Rezaian was denied bail. And for months, he was denied access to proper legal representation, his family has said.
Boxing great Muhammad Ali, also an American Muslim, appealed to Tehran last month to give Rezaian full access to legal representation and free him on bail.
"To my knowledge, Jason is a man of peace and great faith, a man whose dedication and respect for the Iranian people is evident in his work," Ali said in a religiously worded statement.
The journalist has also not been allowed to see visitors aside from his wife and has endured long interrogations, family members have said.
In December, after a 10-hour hearing, Rezaian signed a paper to acknowledge that he understood the charges against him, the Post reported.
Iran's human rights chief, Mohammad Javad Larijani, told news outlet France 24 last year that he hoped Rezaian's case would come to a positive conclusion. He said, "Let us hope that this fiasco will end on good terms."