Italian Rossi, winner of the preceding races in France and China, recovered from a sluggish start from pole to take the lead within four laps.
The five-times premium class world champion, donning a fancy helmet with his own face on top, never let it go and finished over two seconds ahead of Australian world champion Casey Stoner on a Ducati.
Spain's Dani Pedrosa came in third on a Honda, while San Marino's Alex de Angelis, also on a Honda, finished fourth after starting on the fourth row of the grid.