Alonso started from the pole and lost the lead to Jenson Button at the first corner but retook it after the pit stops to hold on for a 2.9-second victory over the McLaren driver.
Alonso's victory in his Monza debut for Ferrari moved him back into Formula One's points race, which is led by Mark Webber who finished sixth. Lewis Hamilton crashed out on the first lap and trails Webber by five points — 187 to 182. Alonso has 166 points.
Felipe Massa of Ferrari was third.