Treasury Secretary Steven Mnuchin said Seif "covertly funneled" money from Iran's Revolutionary Guards through al-Bliad Islamic Bank in Iraq to help Hezbollah.
The U.S. Treasury is also imposing sanctions on the Iraq bank and its chairman.
With this, the Treasury Department is trying to clamp down on Iran's ability "to abuse the US and regional financial systems".
On may 8 the President of the United States Donald trump announced the withdrawal from the agreement on Iran's nuclear program and has announced new sanctions against the Iranian regime.
In the second move in a week taking aim at the money networks of the Revolutionary Guards, or IRGC, the US Treasury also blacklisted another senior official, Ali Tarzali, who works in the central bank's global division.
That deal, reached in 2015 and formally known as the Joint Comprehensive Plan of Action (JCPOA), sought to curb Iran's pursuit of a nuclear arsenal by providing significant sanctions relief for the country.
The statement said the designations "do not extend" to the Central Bank of Iran.
A European official said that some senior figures in the USA administration were pushing for a "North Korea scenario" in Iran, in reference to the drastic sanctions imposed on the North Korean regime which Washington believes helped push the country's leader, Kim Jong Un, into halting missile and nuclear testing and agreeing to renewed negotiations.
The United States on Tuesday designated the head of Iran's central bank a "specially designated global terrorist". The sanctions concentrating on Iran's central financial institution executives are among the first actions by Trump's administration since pulling out of the deal to begin ramping up that financial strain.
It is, however, reflective of Trump Administration eagerness to ramp up sanctions on Iran, and hopes that they can intimidate the world into going along with unilateral USA sanctions. The European members of the deal are trying to keep it alive without the U.S. Yet it's unclear that will be workable, because Trump has vowed to punish European companies that continue doing business with Iran despite re-imposed U.S. sanctions.