"We welcome this responsible step by Iran's government which shows Tehran's intention to move along the road toward ensuring complete transparency of its nuclear program," the foreign ministry said in a statement.
Iranian Ambassador Ali Akbar Salehi signed the protocol to the nuclear Non-Proliferation Treaty (NPT) at a ceremony at the Vienna headquarters of the UN nuclear watchdog agency, the International Atomic Energy Agency (IAEA).
The move was hailed as a watershed for Tehran as it seeks to fend off accusations that it is developing nuclear weapons.
"Iran has turned a new leaf," said IAEA director Mohamed ElBaradei.
Russia is building the Islamic state's first nuclear power reactor at Bushehr, which is due to be completed in 2005, a Russian nuclear ministry spokesman told AFP.
The project has been repeteadly delayed because of negotiations over a separate agreement under which Tehran would pledge to return spent fuel from the plan back to Russia.
The two sides have said they would sign that agreement in January, during a visit to Iran by Russian Atomic Energy Minister Alexander Rumyantsev.