Foreign Minister Manouchehr Mottaki did not say when proceedings would begin or specify the charge other than to say the Americans had "suspicious aims." In November, however, authorities accused the Americans of spying.
There are concerns in the U.S. that Iran could use them as bargaining chips in talks over its nuclear program or in seeking the return of Iranians they say are missing.
Relatives and the U.S government say the three were innocent tourists on an adventure hike in northern Iraq and accidentally crossed into Iran, where they were arrested on July 31.
"They will be tried by Iran's judiciary system and verdicts will be issued," Mottaki said at a news conference, without elaborating in detail. He said the three were still being interrogated.
The Americans - Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27 - were detained by Iranian authorities after crossing an unmarked border from northern Iraq.
Back in September, the brother of Joshua Fattal one of three American hikers detained in Iran told Action News the family hoped that President Mahmoud Ahmadinejad would bring the trio with him when he visited the United Nations. That didn't happen.
They have been held in Iran's Evin prison, where Swiss diplomats have visited them twice and said they are healthy. Because the U.S. and Iran do not have direct diplomatic relations, the Swiss Embassy maintains an American interests section.
The three graduates of the University of California at Berkeley had been trekking in Iraq's northern Kurdistan region, their relatives say.
Bauer and Shourd had been living in Damascus, Syria - he studying Arabic, she teaching English - and both had done freelance journalism or writing online. Friends have described them as passionate adventurers interested in the Middle East and human rights.
Fattal, who spent three years with a group dedicated to sustainable farming near Cottage Grove, Oregon, had been overseas since January as a teaching assistant with the International Honors Program.
Fattal's mother, Laura, declined to comment on Monday's announcement.
U.S. Secretary of State Hillary Rodham Clinton has called for their release, saying Washington strongly believes there is no evidence to support any charge against them.
In November, Tehran chief prosecutor Abbas Jafari Dowlatabadi said the three "have been accused of espionage." But it was not clear from his brief comments whether formal charges had been filed against the Americans.
Raising concerns that Iran might be seeking to use them in a deal, President Mahmoud Ahmadinejad said during remarks about their case last month that the United States was holding several Iranian citizens.
In particular, he drew a link between the case of the three Americans and the trial in the U.S. of Amir Hossein Ardebili, an Iranian who faces up to 140 years in prison after pleading guilty to plotting to ship sensitive U.S. military technology to Iran.
According to court papers, Ardebili worked as a procurement agent for the Iranian government and acquired thousands of components, including military aircraft parts, night vision devices, communications equipment and Kevlar. U.S. federal authorities targeted him in 2004 after he contacted an undercover storefront set up in Philadelphia to investigate illegal arms trafficking.