EU foreign policy chief Catherine Ashton, who coordinates nuclear talks between Iran and world powers, arrived in Tehran on Saturday, media reported.
Her visit comes amid a recent thaw in Iran's strained relations with the West following last year's election of moderate Iranian President Hassan Rouhani and over its controversial nuclear programme.
Rouhani vowed to "constructively engage" with the West, and a historic interim nuclear deal struck in November came into force in January.
The interim deal requires that the Islamic republic curb its nuclear activities for six months in exchange for some sanctions relief.
Negotiators are aiming to reach a comprehensive agreement by July 20, when the interim accord is due to expire.
"Ashton and her political and economic delegation arrived in Tehran and were officially welcomed by Deputy Foreign Minister Abbas Araqchi," official IRNA news agency said.
Araqchi is Tehran's chief nuclear negotiator in the talks with world powers.
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Ashton's visit is the first to Iran by an EU policy chief since 2008.
She is scheduled to meet Rouhani, Parliament speaker Ali Larijani and Foreign Minister Mohammad Javad Zarif on Sunday.
Sources close to her delegation said that later Saturday she was also due to meet representatives of civil society, mainly women.
Ashton's trip -- which follows official visits by top diplomats from Italy, Sweden, Belgium and Spain -- will also take her to Isfahan on Monday, Iranian media reports said.
The European Union is a key player in the nuclear negotiations with Iran, and Ashton is credited as playing a pivotal role.
Talks aimed at reaching a comprehensive and permanent deal are to begin in New York next month.
The United States, other Western powers and Israel have long suspected Iran of using its civil nuclear energy programme as a cover for developing atomic weapons, a charge denied by Tehran.
In return for eased sanctions, Iran undertook to limit enrichment of uranium to five percent purity, halting enrichment to the higher levels that had prompted Western concern on whether its intentions were entirely peaceful.
It also undertook to neutralise existing stockpiles of higher-enriched uranium and to suspend work on a heavy water reactor it had been building.