EU foreign policy chief Catherine Ashton is making her first visit to Tehran.
Iranian media is describing it as a “new chapter” in relations between the Islamic Republic and Europe.
Ashton is expected to meet with Iranian leaders, including President Hassan Rouhani, during her two-day trip.
The country’s disputed nuclear programme is expected to be among the issues up for discussion.
Iran and six world powers, represented by Ashton, reached an interim deal over Tehran’s nuclear activities last year.
Work has been curbed in exchange for a limited easing of sanctions.
Iran wants all restrictions lifted so that it can revive its battered economy.
Negotiations on a final agreement are continuing.