The fresh EU sanctions include a ban on investments, technical assistance and technology transfers to Iran's key oil and gas industry.
On Wednesday, the US announced its own fresh sanctions, implementing the UN measures approved last week.
Western powers suspect Iran is seeking nuclear weapons - which Tehran denies.
The new EU sanctions were approved by a summit in Brussels, diplomatic sources told AFP news agency.
In a statement quoted by Reuters news agency, the leaders expressed regret "that Iran has not taken the many opportunities which have been offered to it to remove the concerns of the international community over the nature of the Iranian nuclear programme".
The sanctions could come into force within weeks, Reuters said.
Correspondents say they will put strong pressure on Iran, which is the world's fifth-largest oil exporter, but has limited refining capability.
The US sanctions announced on Wednesday ban Americans from trading with a number of firms and individuals, including Iran's Post Bank, Defence Minister Ahmad Vahidi and the air force and missile command of the Revolutionary Guard Corps.
On 10 June the Security Council endorsed a fourth round of UN sanctions on Iran, including tighter financial curbs and an expanded arms embargo.
Iranian President Mahmoud Ahmadinejad dismissed the vote as "a used handkerchief" fit for the dustbin.
Tehran has rejected calls by the Security Council to halt uranium enrichment - which could have military as well as civilian uses.
Iran insists its nuclear programme is solely designed to produce energy.