"The only thing I can say is that (the jail sentence) was more than 12 years," lawyer Houshang Azhari told IRNA. Azhari, who did not give further details, said the sentence would be appealed.
U.S. State Department spokesman Ian Kelly urged Tehran to immediately release the scholar, saying the United States was "deeply concerned" about the long jail term. Kelly said he had been told Tajbakhsh had been sentenced to 15 years.
"Mr. Tajbakhsh poses no threat to the Iranian government or to national security," Kelly told reporters. "Given the groundless nature of charges against him, we call on Iran to grant his immediate release."
"He may also have been forced to stand trial in the Revolutionary Court without the benefit of a self-appointed lawyer," Kelly said.
The verdict comes as United States is seeking to engage the Islamic Republic in direct talks to resolve a long-running row over Tehran's disputed nuclear ambitions.
Tajbakhsh was among more than 100 people detained after the presidential poll in August. He was accused of espionage and acting against national security.
Huge opposition protests followed the poll, plunging Iran into its deepest internal crisis since the 1979 Islamic revolution.
The moderate opposition says the vote was rigged to secure President Mahmoud Ahmadinejad's re-election. Officials deny the accusations.
Iranian authorities have portrayed the protests that erupted after the election as a foreign-backed bid to undermine the Islamic Republic's clerical leadership.
In July, the United States called on Iran to release Tajbakhsh and said it was "deeply concerned" about the scholar after he was arrested for a second time in just over two years.
Tajbakhsh, an Iranian American who holds a doctorate in urban planning from Columbia University, was first arrested by Iranian authorities in May 2007, charged with spying and then released after more than four months in Tehran's Evin prison.
The United States, which cut diplomatic ties with Tehran after the revolution, accuses Iran of seeking to develop nuclear weapons. Iran says its nuclear program is to generate electricity so it can export more oil and gas.
(Additional reporting by Deborah Charles in Washington)
(Editing by Cynthia Osterman)
