Two other detainees were transferred to Pakistan and a fourth to Bahrain, the Pentagon said.
A Pentagon statement did not identify the four or provide specifics about the Iranian transfer.
"These detainees were all recommended for transfer due to Administrative Review Board processes conducted at Guantanamo Bay," the statement said.
It said about 110 other detainees were awaiting release or transfer as a result of various reviews, and more movements of prisoners from the detention center were expected.
About 440 detainees are held at the facility, located at a US naval base on the south-eastern tip of Cuba.

Copyright AFP (Agence France-Presse), 2006