The Blackwater Worldwide contractors - Nicholas Slatten, Paul Slough, Evan Liberty and Dustin Heard - were among 15 people that the outgoing president pardoned in his last weeks at the White House.
The incident caused an international uproar over the use of private security guards in a war zone.
But supporters of the four had lobbied for pardons for the men who were all serving lengthy prison sentences.
They argued the men had been excessively punished in an investigation and prosecution they said was tainted by problems and withheld exculpatory evidence.
Also receiving pardons were two men convicted of lying to investigators leading the probe into Russian interference with the Trump election campaign.
George Papadopoulos, Mr Trump’s 2016 campaign adviser whose conversation unwittingly helped trigger the Russia investigation that shadowed Mr Trump’s presidency for nearly two years, was among those pardoned.
He also pardoned Alex van der Zwaan, a Dutch lawyer who was sentenced to 30 days in prison for lying to investigators during special counsel Robert Mueller’s investigation.
Mr Trump’s actions in his final weeks in office show a president who is wielding his executive power to reward loyalists and others who he believes have been wronged by a legal system he sees as biased against him and his allies.
He issued the pardons – not an unusual act for an outgoing president – even as he refused to publicly acknowledge his election loss to Democrat Joe Biden, who will be sworn in on January 20.
Last month, Mr Trump pardoned former national security adviser Michael Flynn, who had twice pleaded guilty to lying to the FBI, and months earlier commuted the sentence of another associate, Roger Stone, days before he was to report to prison.