The military says in a statement released Wednesday that the alleged member of Iran's Revolutionary Guards' elite Quds force was detained at Baghdad Airport while trying to leave the country on Tuesday.
The military says the suspect was using a job building and repairing religious sites in Iraq as cover to funnel weapons into the country.
The U.S. alleges the Iranian was smuggling weapons into Iraq in legitimate shipments of building materials.
The U.S. has long accused Iran of training and arming Shiite extremist groups in Iraq and fueling the insurgency.
Tehran denies the charges.