Navi Pillay, the U.N.'s high commissioner for human rights, said she based her tally on a review of Iranian press reports. Tehran does not officially publish the number of its executions.
"We have urged Iran, time and again, to halt executions," Pillay said. "I am very dismayed that instead of heeding our calls, the Iranian authorities appear to have stepped up the use of the death penalty."
Most executions were for drug offenses, she said, but at least three were for political activism. Two executions were held in public, which Pillay said compounded their cruelty and inhumanity.
Iran's U.N. mission in Geneva declined requests for comment Wednesday.
Pillay said she was worried about the large number of political prisoners, drug offenders and even juvenile offenders who remain on death row in Iran.
She expressed particular concern over three cases in which political activists were executed: Jafar Kazemi, Mohammad Ali Haj Aqaei and another man whose identity the U.N. said was not disclosed.
"Dissent is not a crime," Pillay said. "It is absolutely unacceptable for individuals to be imprisoned for association with opposition groups, let alone be executed for their political views or affiliations."
Kazemi and Haj Aqaei were executed because they were members of the exiled opposition movement, the Mujahedeen-e Khalq Organization, or MEK, said Shahin Gobadi, spokesman for the Paris-based National Council of Resistance of Iran, which speaks for MEK.
MEK, a formerly armed group largely wiped out in Iran in the late 1980s, is also known as the People's Mujahedeen Organization of Iran. It became allied with Saddam Hussein's regime in Iraq, which helped fund the group's attacks against the Iranian regime.
Some MEK members are among the Iranian dissidents living at Camp Ashraf, northeast of Baghdad. Following the U.S. invasion of Iraq in 2003, the Americans disarmed several thousand MEK members and promised to protect them at Ashraf.
Maryam Rajavi, president-elect of the National Council of Resistance of Iran, called on the U.N. Security Council to impose more sanctions on Iran to address its human rights violations. Iran is under U.N. sanctions for its nuclear activities.
The U.S. considers MEK a terrorist organization. The European Union removed it from its terror list in 2009.
Gobadi said Kazemi and Haj Aqaei - hanged last month after being arrested in September 2009 - had visited Camp Ashraf recently and "that was their sin" in the eyes of Iranian authorities.
The National Council of Resistance of Iran has been urging the European Union to cripple Iran's economy through embargoes on the Iranian Central Bank and by banning purchases of Iranian oil and gas.
The Dutch government froze official contacts with Iran this week to protest the hanging of a Dutch-Iranian woman, 45-year-old Zahra Bahrami, in Tehran on Saturday.
Iranian state television reported Bahrami was hanged for possessing and selling drugs. She had been jailed since December 2009 after protests against President Mahmoud Ahmadinejad's re-election.
(This version corrects that EU has removed MEK from its list of terror groups in 2009.)