“I learned with dismay of another cowardly attack which has one again hit the city of New York, affecting innocent victims,” Mr Mattarella said.
“In these tragic circumstances, I wish to confirm to you that Italy will always stand by the United States and all countries working to fight the ferocious and blind violence of terrorism and to defend the fundamental values of freedom and cohabitation for which New York is a recognised symbol in the world.”
The attack unfolded on Tuesday afternoon when Uzbek immigrant, Sayfullo Saipov, allegedly used a pickup truck rented from a New Jersey Home Depot to mow down pedestrians and cyclists along the Manhattan bike path, before crashing into a school bus.
He got out of the truck shouting “Allahu Akbar” (Arabic for “God is greatest”), and holding up what turned out to be a paint-ball gun and a pellet gun, authorities said, before a police officer shot him.
New York Police Commissioner James O'Neill said six people were pronounced dead at the scene, and two other people died later in hospital.
The victims were later identified as five Argentine tourists, a Belgian, a New Yorker and a New Jersey man.
Several others were injured and taken to hospital.
Mr Saipov, 29, faces two charges, Manhattan acting US Attorney Joon Kim said.
The charges are one count of violence and destruction of motor vehicles causing the deaths of eight people and one count of providing material support and resources to a foreign terrorist organisation.
The maximum penalty for the former is death, while the maximum for the latter is life in prison, Mr Kim said.
With ANSA
