The Treasury Department said SAD Import Export Company shipped weapons to the Syrian Armed Forces on behalf of Iran's Defense Industries Organization, which it accused of being used by the government to "assist" the Syrian regime's crackdown.
In 21 months, the anti-regime revolt in Syria has claimed more than 44,000 lives, according to monitors.
SAD Import Export and two others, Chemical Industries and Development of Materials Group and Marine Industries Organization, were also blacklisted for their ties to Iran's Defense Industries Organization and its defense ministry.
And the director of the Marine Industries Organization, Mustafa Esbati, was implicated for his support to Iran's Aerospace Industries Organization, as was a fourth company, Doostan International Company.
The executive order targeted "proliferators of weapons of mass destruction and their supporters," and freezes any assets those designated may have under US jurisdiction.
It also puts foreign banks on notice that they can lose access to the US financial system if they "facilitate significant transactions or provide significant financial services" to them.
"We will continue to expose the companies and individuals involved in Iran's illicit weapons program," said David Cohen, undersecretary for terrorism and financial intelligence at the Treasury.
A day earlier, the UN Security Council had slapped sanctions on two Iranian firms accused of supplying arms to the Syrian government -- including SAD Import Export Company -- adding them to a growing Iran sanctions list for breaching an arms embargo ordered against the Islamic republic for its nuclear drive.