The groups, backed and trained by Tehran, delivered a statement from the home of Iran's consul-general in the Shi'ite holy city of Najaf.
They announced their solidarity with the Muslim people and the Revolutionary Guards who they said helped to prevent four or five states from falling to Islamic State militants.
President Donald Trump said last week he would name Iran's elite Islamic Revolutionary Guard Corps a terrorist organization.
The action by Trump, who has taken a hard line toward Iran by withdrawing from the 2015 Iran nuclear deal and re-imposing broad economic sanctions, marks the first time the United States has formally labeled another country's military a terrorist group.
"This is laughable coming from the number one sponsor of terrorism, America," said a spokesman for the Badr Organisation, without providing evidence.
Badr is led by Iraqi militia commander and politician Hadi al-Ameri who's Fateh coalition of militia groups has the second-largest number of seats in Iraq's parliament.
"We reject this action from America and say we have honor to be in the Islamic resistance that fought and beat terrorism," a spokesman for the wider Fateh coalition said.
Some of the militia groups themselves are designated as terrorist organizations by Washington.
Iraqi Prime Minister Adel Abdul Mahdi said on Tuesday that Washington's decision to designate the IRGC a terrorist group could harm his country and the wider region.
Iraqi security forces declared victory over Islamic State in 2017 with help from a U.S.-led coalition and Iran-backed Shi'ite militias, some of which fought U.S. troops earlier following the 2003 U.S.-led invasion that toppled Saddam Hussein.