"Mr. Tajbakhsh poses no threat to the Iranian government or its national security," Kelly said. "Given the groundless nature of charges against him we call on Iran to grant his immediate release.
As an independent, internationally respected academic, Mr. Tajbakhsh has always sought to foster better understanding between Iran and the United States and Iran and the international community."
Kelly said the prison sentence is for 15 years. Iran's state-run news agency, IRNA, reported that Tajbakhsh had been sentenced to more than 12 years in prison.
Tajbakhsh's attorney said he plans to appeal the verdict from Iran's Revolutionary Court, IRNA reported.
Tajbakhsh was arrested in July and was one of more than 1,000 people detained following a massive government crackdown. He was tried along with nearly 100 others, including journalists, reformist leaders and former government ministers.
Exactly what Tajbakhsh was convicted of was unclear. He had been accused of numerous charges, including plotting a "soft revolution" against the Iranian regime through his work with George Soros' Open Society Institute, according to IRNA.
Tajbakhsh resigned from his position with the institute after he was arrested and detained for four months in 2007 on unspecified charges. He holds dual citizenship in Iran and the United States.
The mass trial included Tehran-based Newsweek journalist Maziar Bahari, who was released from prison on Saturday.
Bahari, who is an Iranian and Canadian national, arrived in London, England, on Tuesday to witness the upcoming birth of his first child, the magazine reported.
Secretary of State Hillary Clinton called the mass trial a "sign of weakness" that shows that the Islamic republic "is afraid of its own people."
"It is a show trial, there's no doubt about it," Clinton told CNN's Fareed Zakaria on his "GPS" program. "It demonstrates I think better than any of us could ever say that this Iranian leadership is afraid of their own people, and afraid of the truth and the facts coming out."