Alex Fiva of Switzerland, who won silver four years ago, took bronze this time under challenging conditions in the Alpine town of Livigno. Heavy snow fell throughout the events, limiting visibility and slowing down skiers on the winding race course.
The Italian crowd erupted into huge cheers as Deromedis and Tomasoni sprinted across the finish line. The two men collapsed on their backs to catch their breath in the snow.
At the medal ceremony, Deromedis jumped onto the podium, smiled widely and hugged his teammate. Tomasoni kissed his medal and looked to the sky as spectators waved red-white-and-green Italian flags.
The skiers and crowd then sang the national anthem.
Ski cross features high-speed races between four riders who slide down a 1.5 km course filled with bumps, jumps and turns. The sport is known for being unpredictable, with competitors often knocking into each other and crashing.
Defending gold medallist Rya Regez of Switzerland was disqualified in the semi-finals when judges ruled he interfered with Japan's Satoshi Furuno.
Daniela Maier of Germany claimed the gold medal in the women's ski cross on Friday.