He said the field where United Airlines Flight 93 crashed, after passengers overpowered the hijackers who had commandeered it, "is now a monument to American defiance", and, he said, "a message to the world America will never ever submit to tyranny".
A total of nearly 3,000 people were killed on September 11, 2001.
Almost 3,000 people died in the attacks on 9/11, when worldwide terrorism hit home in a way it previously hadnt for many Americans.
Commemorations also took place in NY and Washington to mark the anniversary of the 9/11 attacks by al Qaeda, in which almost 3,000 people were killed.
Trump and his wife, Melania, traveled to the Flight 93 National Memorial from Washington and paused for a moment of reflection while overlooking the field where the plane crashed.
A citywide moment of silence will also be observed throughout the morning.
White House spokeswoman Sarah Sanders said Mr. Trump would put the emphasis on "remembering that horrific day. and certainly honoring the individuals who were not only lost that day, but also put their lives on the line to help in that process".
Having Trump as president feels like having an abusive stepdad who ruins every function with his ineptitude, except 1000 times worse because his ineptitude is steering one of the most powerful nations in the world.
Memorials to 9/11 continue to grow at Shanksville, where a newly dedicated Tower of Voices will eventually include a wind chime for each of the 40 people killed there, and ground zero, where work is to begin soon on a pathway honoring rescue and recovery workers.
As passengers and crew tried to enter the cockpit to stop the hijackers, the terrorists then made the decision to crash the plane before reaching their final destination - which is believed to have been the Capitol.
The president also participated in the Pentagon's september 11 observance previous year.
Vice President Mike Pence is attending a ceremony at the Pentagon. The annual 9/11 commemorations are by now familiar rituals, centered on reading the names of the dead.
Meanwhile, a subway station destroyed on 9/11 finally reopened Saturday.
Trump has said when talking about Muslims that "thousands of people were cheering" in Jersey City, New Jersey, situated across the Hudson River from lower Manhattan, as the towers collapsed.