The treasury also blocked any US assets of al-Nour Radio and Lebanese Media Group, which it said was the parent company of al-Manar and al-Nour.
It said both media outlets had facilitated the activities of Hizb Allah, the Lebanese resistance group which the US brands a terrorist organisation, including giving support by fundraising and recruitment.
The treasury's action prohibits transactions between Americans and the companies in addition to freezing any assets they may have under US jurisdiction.
Stuart Levey, under-secretary for terrorism and financial intelligence at the treasury, said: "Any entity maintained by a terrorist group - whether masquerading as a charity, a business, or a media outlet - is as culpable as the terrorist group itself."
A treasury statement said al-Manar had also provided support to Palestinian groups defined as terrorist by the US government, including by transfer of tens of millions of dollars to a charity linked to the Palestinian Islamic Jihad resistance movement.
The US state department placed al-Manar on its terrorist exclusion list in December 2004, which meant foreigners providing support to or associated with the organisation could be prevented from entering the US or be deported.
Lebanese officials called that designation unacceptable censorship and an attack on freedom of speech. There was no immediate response from Lebanon to Thursday's announcement by the treasury.