Nasrallah and two other Hezbollah leaders were sanctioned for supporting the Syrian government or for supporting "terrorist activities in the Middle East and around the world," the Treasury Department said.
David Cohen, U.S. undersecretary for terrorism and financial intelligence, said Syria has been a steadfast ally of Hezbollah and the Shiite movement in Lebanon was now returning the favor.
"By aiding (Syrian President Bashar) Assad's violent campaign against the Syrian people and working to support a regime that will inevitably fall, Hezbollah's ongoing activity undermines regional stability and poses a direct threat to Lebanon's security," he said in a statement.
Nasrallah in July said he only solution to the crisis in Syria was through political dialogue but expressed confidence with the Syrian military's ability to control the situation.
Cohen said that, under Nasrallah's direction, Hezbollah is providing training and logistical support to Syrian forces.
There was no immediate reaction from Hezbollah.
Former Lebanese Prime Minister Saad Hariri said during meetings with French officials in Paris that Hezbollah is playing "an immature role" in the Syrian conflict, The Daily Star newspaper in Lebanon reports.
Hezbollah isn't universally considered a terrorist organization as some governments make a distinction between its political and military wings.