Trump said on Tuesday that the US "shouldn't have destabilized" Iraq before pivoting to praising Saddam Hussein.
"He was a bad guy -- really bad guy. But you know what? He did well. He killed terrorists. He did that so good. They didn't read them the rights. They didn't talk. They were terrorists. Over. Today, Iraq is Harvard for terrorism," Trump said as cited by CNN on Wednesday.
The remarks came during a rally where he was introduced by Tennessee Sen. Bob Corker, chairman of the Senate Foreign Relations Committee, who is a potential running mate for Trump.
Asked Tuesday night on Fox News about the comments, House Speaker Paul Ryan appeared taken aback by Trump's words.
"He was one of the 20th century's most evil people," Ryan said of the former Iraqi dictator.
The Clinton campaign jumped on the remarks, with senior campaign adviser Jake Sullivan saying "Trump's praise for brutal strongmen seemingly knows no bounds."
"Trump yet again lauded Saddam Hussein as a great killer of terrorists, noting with approval that he never bothered to read anyone their rights," Sullivan said in a statement.
The presumptive Republican nominee has previously said that Iraq and Libya, two countries parts of which are under Daesh (also known as ISIL and ISIS) control, would be better off if Saddam Hussein and Libyan dictator Moammar Gadhafi were still alive and in power in their respective countries.