Hashem Safieddine is head of the executive council of Hezbollah, an Iranian-backed Lebanese armed movement which Washington has branded a "foreign terrorist organisation".
"The Kingdom of Saudi Arabia joined the United States in designating Hashem Safieddine," the State Department said in a statement.
"As a result, any of his assets held in Saudi Arabia are frozen, and transfers through the Kingdom's financial sector, are prohibited."
Separately, the department's Bureau of Counterterrorism tweeted that this marked the "first-ever" State Department and foreign nation "joint terrorist designation", underlining the close cooperation between US and Saudi officials.
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The official Saudi news agency SPA confirmed Safieddine's listing, and alleged he had given his organisation advice on carrying out terrorist acts and on supplying support to Syrian President Bashar al-Assad's regime.
Trump has chosen the kingdom as the venue of his first foreign presidential visit, and this weekend he will meet King Salman and address an audience of up to 50 leaders from across the Muslim world on the threat of extremism.
He is a cousin of Hezbollah's overall leader Hassan Nasrallah, and is spoken of a potential candidate to succeed him and take charge of perhaps the most powerful non-state movement in an unstable region.
The US designation order did not link him to any recent Hezbollah attacks, but noted the group's historical involvement in the 1983 bombing of a US Marines barracks in Beirut, a US embassy bombing in 1984 and a passenger jet hijacking in 1985.
As "specially designated global terrorists", Safieddine and al-Isawi will see any assets they hold in areas under US jurisdiction frozen, and US citizens will be forbidden from any dealings with them.
Separately but simultaneously, the US Treasury added two Yemeni tribal leaders, Hashim Muhsin Aydarus al-Hamid and Khalid Ali Mabkhut al-Aradah, to its own sanctions list, branding them leaders of Al-Qaeda in the Arabian Peninsula.