"In essence the results are cooked. They are cooked in the sense that the Iranian people were not able to vote for a full range of people," said State Department spokesman Sean McCormack of the poll.
Iranians voted on Friday in an election likely to keep parliament in the control of conservatives after unelected state bodies barred many reformist foes of President Mahmoud Ahmadinejad from the race.
"They are given the choice of choosing between one supporter of the regime or another supporter of the regime.
They were not given the opportunity ... to vote for somebody who might have had different ideas," McCormack told reporters of the poll.
Ahmadinejad has shrugged off criticism about the poll.
McCormack said the real power in Iran was held by an "unelected few," with Iran's Supreme Leader Ayatollah Ali Khamenei at the top.
"These are not constituent elements of a thriving democracy and that is a shame for the Iranian people that they are denied the ability to choose truly who will lead them and be able to freely express their choices through the ballot box," added McCormack.
Last week, the United Nations Security Council passed a new round of sanctions against Iran for its refusal to give up sensitive nuclear work that the West suspects is aimed at building an atomic bomb. Iran argues it is for the generation of electricity.
The United States and other major powers are expected to meet in the coming weeks to discuss how to proceed against Iran, with Russia and China urging greater incentives to get Tehran to change its behaviour.