Abd ar-Rahman Mustafa al-Qaduli, aka Haji Imam, was a known terrorist who was said to be the group's second in command and minister of finance.
The terror leader died in a clandestine raid conducted on the ground in Syria by U.S. Special Operations Forces, military sources told ABC News.
Such deadly operations come in the wake of a terror attack in Belgium this week that left at least 31 dead and 300 injured, for which ISIS has claimed responsibility.
Al-Qaduli had been listed on the State Department's Rewards for Justice website, with a bounty of $7 million. He is described there as a senior leader of ISIS with a long history of terrorism, joining al-Qaeda in 2004 and serving as a deputy to now deceased al-Qaeda in Iraq leader Abu Musab al-Zarqawi. He was released from an Iraqi prison in 2012 shortly after the withdrawal of U.S. forces.
"We are systemically eliminating ISIL's cabinet," Secretary Carter said, using an alternate acronym for ISIS. "Indeed, the U.S. military killed several key ISIL terrorists this week, including, we believe, Haji Imam, who was an ISIL senior leader, serving as a finance minister and who is also responsible for some external affairs and plots.
"The removal of this ISIL leader will hamper the organization's ability for them to conduct operations both inside and outside of Iraq and Syria," Carter said.
Pentagon officials earlier this month said they were confident senior ISIS military commander Abu Omar al-Shishani, known as Omar the Chechen, had died from injuries sustained in a March 4 airstrike in northeast Syria.