First Lady Melania Trump accompanied him on the surprise trip.
Trump said he has “no plans at all” to order a withdrawal of American forces from Iraq, similar to what he ordered for US troops in Syria last week.
He had previously said that US forces could use Iraq as a staging ground for targeted forays across the border into Syria.
There are some 5,000 US soldiers in Iraq, compared to about 2,000 on the ground in Syria.
Trump spent about three and a half hours on the ground, visiting troops, meeting US officials, and taking a few questions from reporters.
A scheduled meeting with Iraqi Prime Minister Adil Abdul-Mahdi was cancelled. It is not immediately clear why.
He defended his decision to withdraw from Syria, saying "The United States cannot continue to be the policeman of the world."
.@FLOTUS Melania and I were honored to visit our incredible troops at Al Asad Air Base in Iraq. GOD BLESS THE U.S.A.! pic.twitter.com/rDlhITDvm1
— Donald J. Trump (@realDonaldTrump) December 26, 2018
Updated at 11:58 pm
