Trade officials said on Thursday that the United States, which repeatedly had blocked Iran's long-held desire to join the WTO, raised no objections.
The WTO takes such decisions by consensus.
"This is an historical decision," said Ujal Singh Bhatia, India's ambassador to the WTO.
"The principle of universality has been strengthened. We look forward to working with Iran for its accession."
As a result of the decision on Thursday, Iran now has observer status and can sit in all meetings of the WTO.
Iran first applied to join the WTO in 1996, but the US blocked its application 22 times.
US officials in Geneva declined to comment on the decision made by the WTO's governing General Council.
WTO membership is one of the rewards European Union negotiations have been offering Iran if it agrees to curb its nuclear program to ensure that it produces only electricity and not weapons.