Here's what you need to know.
It turns out, though, that the Mammadovs, the Azerbaijani family with whom the Trump Organization worked with on the development was financially entangled with an Iranian family whose construction company is controlled by the IRGC.
The unprecedented move is part of the Trump administration's broader effort to counter Iranian terrorism around the world, the announcement said.
There are more than 150,000 troops in the IRGC, according to the Council of Foreign Relations.
The decision came after Foreign Minister Mohammad Javad Zarif urged President Hassan Rohani to place CENTCOM on Tehran's list of "terrorist" groups.
In 2009, the Guards' Basij branch - one of five IRGC subgroups- cracked down hard on anti-government demonstrations in the wake of a contested election, helping to secure the position of hardline then-President Mahmoud Ahmadinejad.
The Revolutionary Guards have always been in the crosshairs of successive U.S. administrations.
Secretary of State Mike Pompeo has pushed for the change in US policy as part of the Trump administration's tough stance toward Tehran.
It added that Washington's "unwise and illegal measure is a major threat to regional and worldwide stability and peace".
The decision comes hours before Israeli Prime Minister Benjamin Netanyahu faces re-election in tight polls.
Turkey criticizes State Department over description of Pompeo meeting
Meanwhile, Russia and Turkey remain in talks over the delivery date for S-400 parts, originally slated for May. Turkish Vice President Fuat Oktay responded on Twitter to Pence's ultimatum: "The United States must choose".
As Washington claimed, the Revolutionary Guards were providing assistance to the Syrian military, as well as backing the activities of the Hezbollah and Hamas, which the United States and some of its allies consider to be terrorist groups.
Read the full report at Fox News. Speaking to reporters, he rattled off a list of attacks dating to the 1980s for which the US holds Iran and the IRGC responsible, beginning with the attacks on the Marine Corps barracks in Beirut, Lebanon in 1983. "This accounts for 17 percent of all deaths of USA personnel in Iraq from 2003 and 2011".
Moreover, the designation has the potential to stir up greater conflict and instability in a region that is already rife with both.
Experts believe the designation could bring the USA and Iran ever closer to a direct military confrontation and dash any hopes of reviving talks over the 2015 nuclear deal.
Iran retaliated by declaring U.S. forces in the Middle East as a terrorist organisation, Iran state news reported.
The Trump administration has adopted a hardball stand towards terrorism affecting the USA and its close allies, including taking a tough stand towards Pakistan despite Washington needing Islamabad help in Afghanistan.
"It makes crystal clear the risks of conducting business with, or providing support to, the IRGC", Trump said.
Revolutionary Guards chief Jafari has already promised retaliation.
The designation marks the first time Washington has branded a foreign government official military entity a terrorist group, and is certain to engender thoughts in India to seek a similar designation for the Pakistani military, particularly its intelligence unit ISI, which New Delhi says has always been involved in terrorist activities in India.
The designation represents the strongest salvo yet by the Trump administration to choke off Iran's global resources and target its regional terror operations, which continue almost unfettered in hotspots such as Yemen, Syria, and Iraq. "Thank you for answering another one of my important requests, which serves the interests of our country and the countries of the region".