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".
The 9/11 plane strikes, which destroyed the Twin Towers and left a gaping hole in the Pentagon, killed almost 3,000 people, majority in Manhattan.
Americans looked back on 9/11 Tuesday with solemn ceremonies, volunteer service and a presidential tribute to "the moment when America fought back" on one of the hijacked planes used as weapons in the deadliest terror attack on US soil.
U.S. President Donald Trump and first lady Melania Trump walk with park superintendent Stephen Clark at the Flight 93 National Memorial during the 17th annual September 11 observance at the memorial near Shanksville, Pennsylvania, U.S., September 11, 2018.
In New York City, at the National September 11 Memorial & Museum, relatives of those killed in the collapse of the Twin Towers continued the tradition of reading the names of the victims as church bells tolled on the moments the planes struck.
- President Donald Trump is marking 17 years since the worst terrorist attack on USA soil by visiting the Pennsylvania field that became a September 11 memorial.
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".
A stark reminder came not long after last year's anniversary: A truck mowed down people, killing eight, on a bike path within a few blocks of the World Trade Center on Halloween. It is the place where the fourth planned attack was foiled, when 40 passengers and crew members stormed the cockpit hoping to stop the hijackers. The 93ft tall structure features 40 wind chimes of different sizes.
Mr Trump is the third sitting president to attend the crash site, 70 miles (110km) southeast of Pittsburgh.
At 10:03 am on September 11, 2001, the moment when the Boeing jetliner crashed in an enormous plume of smoke, the names of the victims will be read one by one.
The gathering was attended by the New York City Police Department, as well as the relatives of victims, according to a report by the Associated Press.