The president confirmed his plans to reporters on Friday - but would not say why he is skipping the ceremony in New York, which is being held at the site where the twin skyscrapers stood.
"Because I'm going to the Pentagon," he replied, according to USA Today.
According to the White House, Vice President JD Vance will attend the memorial service in New York. Since 2012, politicians have been barred from speaking at the ceremony there, as organisers want it to be completely apolitical.
During the 2001 attacks, terrorists hijacked commercial airliners and crashed them into the World Trade Center in New York and the Pentagon near Washington, D.C. Nearly 3,000 people were killed.
