Iran has signed an agreement with the United Nations which will allow tougher inspections of its nuclear industry.
Iran first agreed to sign the protocol in October
The Iranians have consented to the measure amid concern that they are trying to produce nuclear bombs - an allegation the Islamic republic denies.
The UN's nuclear watchdog will receive more access to data and greater powers, although snap inspections - such as those in pre-war Iraq - are ruled out.
The US has cautiously welcomed the move but says it is only a first step.
Iran signed the Additional Protocol to the 1968 international nuclear Non-Proliferation Treaty at International Atomic Energy Agency (IAEA) headquarters in Vienna.
The document was signed by Tehran's outgoing representative to the agency, Ali Akbar Salehi, and also by IAEA chief Mohamed ElBaradei.
An IAEA report last month, based on limited inspections, found no evidence that nuclear weapons were being developed but did accuse Iran of concealing some of its nuclear research.
The BBC's Jim Muir in Tehran notes that the signing of the protocol was a formality as Iran had agreed earlier this year to act as though it was already in force. Since then, it has been allowing the IAEA's inspectors all the access they have requested.
Yet the protocol does have great symbolic and legal significance, our correspondent adds.
Mr ElBaradei called the signing an "important building block
toward establishing confidence that Iran's programme is
exclusively for peaceful purposes".
The signing of the protocol marks the culmination of a process which over 10 months has seen Iran moving towards greater compliance with the IAEA to avoid the prospect of UN Security Council sanctions.
THE ADDITIONAL PROTOCOL
IAEA has right to conduct searches at shorter notice - but not unannounced inspections
Inspectors get broader search powers including the right to demand information on non-government facilities and the right to check radiation levels in wider areas
Government has greater obligation to report all matters relating to its nuclear fuel cycle
Source: UN agencies
The kind of inspections carried out under its provisions are not random and intrusive, which was one of Iran's concerns.
Any suspicions about possible undeclared sites have to be made known to the host government, which is given every opportunity to clarify the situation, and inspections are only insisted upon as a last resort.
The question of whether Iran should allow tougher inspections sparked intense debate in the country earlier this year with conservatives arguing the inspections would allow spies into the country.
Tehran has insisted that its nuclear programme, which is largely based on Russian technology, is peaceful and that it is needed to meet a growing energy demand although critics point to its huge existing energy reserves, such as oil.
In October, Iran acceded to pressure during a visit by the foreign ministers of Britain, France and Germany and agreed to sign the protocol and suspend uranium enrichment.
Iran has indicated the protocol will have to go through a long process of ratification involving the government, parliament and religious Guardians' Council.
However, Mr ElBaradei said he had been assured Iran would act as though the protocol was already in force.
The US State Department has welcomed Iran's acceptance of the protocol but said it was "only one step" towards resolving questions about the nature of its nuclear programme.