The announcement was made Monday by the National Park Foundation, the official charity of America's national parks.
The pop vocalist will sing at the Sept. 10 event near Shanksville that will include dignitaries and relatives of those killed on United Flight 93.
Forty passengers and crew died on Sept. 11, 2001, after fighting with hijackers aboard the plane, which crashed in a field.
Organizers have raised about $52 million of the approximately $60 million needed for the "Tower of Voices" memorial, which is designed to house 40 wind chimes.
McLachlan has won Grammy Awards for her songs "Building a Mystery" and "I Will Remember You."