Harper is in New York to commemorate the anniversary of the day when hijacked jetliners were flown into the World Trade Center in Manhattan and the Pentagon in Washington.
Harper called them "senseless and cowardly attacks" that took nearly 3,000 lives, including 24 Canadians.
He said Canadians share in the grief of all those mourning loved ones lost today.
Harper paid tribute to the acts of courage, sacrifice and kindness by those who took part in rescue efforts.
And the prime minister said: “While we honour and remember those who fell, this day will serve as a constant reminder that we are not immune from terrorism."
"We will continue to stand firm with our allies to help ensure such a tragedy never happens again," he said.
“Terrorism will not undermine our way of life."
"We will continue supporting the brave Canadian Armed Forces members and intelligence and police officers who put their lives on the line every day in the fight against the many faces of terrorism."
"We will steadfastly defend, protect and promote our democratic values and principles; the very foundation of our free and prosperous society."
“It is what the victims would expect and what the families deserve.”