President Sergio Mattarella mourned the Bastille Day massacre and said Italy and the international community would not give in to terror.
"The horror, the grief of France are our horror, our grief. The dead in Nice, of whatever nationality, are our dead," Mattarella said.
"We will never give in to those who preach and practice the culture of death," the president added.
"Along with...the entire international community [and] especially the European Union, we will continue to affirm our commitment against violence and terrorism and to uphold the values of freedom, equality and brotherhood celebrated in France on July 14".
Premier Matteo Renzi also expressed Italy's commitment to standing with France.
"Today more than ever before, we stand with our brothers across the Alps," the Italian premier said.
Terrorists "will never win" athough the images of the Nice massacre "continue to reverberate in our heads and hurt us", Renzi wrote in his online newsletter earlier in the day.
French and Italian flags flew at half-mast outside the premier's office in Rome's Chigi Palace as a sign of mourning and solidarity.
Civic flags were also flown at half-mast in Milan.
Rome city hall will be lit up with the colours of the French flag on Friday night in solidarity with the victims.
Rome Mayor Virginia Raggi has sent messages of condolence to her counterpart in the French Riviera city Philippe Prada and to the French ambassador in Rome, Catherine Colonna.
Pope Francis also expressed his solidarity with France and the victims of Thursday evening's terrorist attack in Nice, condemning "every manifestation of homicidal folly, hatred, terrorism, and attacks against peace".
"We have followed during the night, with the greatest concern, the terrible news which has come from Nice," Vatican spokesman Father Federico Lombardi said.
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