Al-Harzi played a key logistical role for ISIS, overseeing efforts to get jihadists and weapons into Iraq and Syria to fuel the militant organization's war machine, according to U.S. authorities.
The Tunisian, believed to be one of the first foreign fighters to join ISIS, also helped organize the Sunni extremist group's devastating use of suicide and vehicle-borne bombs in Iraq, the U.S. Department of Defense said.
"This was a big get," said Mike Rogers, a CNN national security commentator and former chairman of the U.S. House Intelligence Committee. "It will be very disruptive to their operation for at least some period of time."
$3 million reward
Al-Harzi was killed in Shaddadi, Syria, on June 16, said Capt. Jeff Davis, a spokesman for the Department of Defense.
The ISIS leader had been on the U.S. Designated Terrorist List since last year and the State Department had offered a $3 million reward for information on him.
The Pentagon said last month that al-Harzi's brother, who was also an ISIS operative, was killed in a U.S. airstrike in Mosul, Iraq, on June 15.
The brother, Ali Awni al-Harzi, was seen as an intermediary between ISIS and jihadists in North Africa. The U.S. government labeled him "a person of interest" in the deadly attack on the U.S. consulate in Benghazi, Libya, on Sept 11, 2012.
Role beyond Iraq and Syria
But Tariq al-Harzi was a more prominent figure in ISIS.
He was believed to be in charge of the terrorist group's operations beyond Iraq and Syria -- operations that have become much more significant in recent months as ISIS has established a foothold in Libya and gained affiliates in Nigeria and Egypt, among other places.
He organized the procurement and shipping of weapons to Syria from Libya, the Pentagon said.
According to a jihadist profile, Tariq al-Harzi was freed from Abu Ghraib prison during an ISIS raid in July 2013.
The United States has targeted other senior ISIS leaders. In May, an Army Delta Force raid in eastern Syria killed Abu Sayyaf
, who U.S. officials said was in charge of oil and gas financing for the militant group, as well as other operations.