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 on Thursday by a summit in Brussels.
In a statement, 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".
Iran's shipping and air cargo companies will also be banned from operating in EU territory, and new visa bans and asset freezes will target Iranian Revolutionary Guards.
The new EU sanctions will now be passed on to experts who will work out which companies and products would be targeted.
Correspondents say the move will put strong pressure on Iran, which is the world's fifth-largest oil exporter, but has limited refining capability.
On Thursday Russia criticised the EU's planned sanctions.
"We are extremely disappointed that neither the United States nor the European Union is heeding our calls to refrain from such steps," Russian news agencies quoted Deputy Foreign Minister Sergei Ryabkov as saying.