The observance begins with the tolling of a bell and a moment of silence, renewing a tradition that has brought families, survivors and public officials together for decades.
Family members are expected to read the names of those who died in the attacks on the World Trade Center, at the Pentagon and aboard Flight 93.
The ceremony also remembers the six people killed in the 1993 World Trade Center bombing, whose names are included in the annual tribute.
Events in New York form part of a broader national day of remembrance, with ceremonies planned at the Pentagon and the Flight 93 National Memorial in Pennsylvania.