Astana rider Aru made his move with 2.5 kilometres (1.55 miles) remaining and finished 21 seconds ahead of Fabio Duarte. Pre-race favourite Nairo Quintana edged out Pierre Rolland for third place, 22 seconds behind.
“I’m very happy,” a tearful Aru said after crossing the line. “It’s thanks to my team-mates, who have been amazing not only today but all race. This win is for them. It’s really pleasing but I still have a lot to learn and to grow. I still can’t believe it.”
The 23-year-old Aru also cut into Rigoberto Uran’s overall lead and is now fourth – two minutes, 24 seconds behind the Colombian and a credible contender.
Uran extended his advantage over his closest challengers and is 1:03 ahead of Cadel Evans, the 2011 Tour de France champion, and 1:50 ahead of Rafal Majka.
“It was a long day and the teams were very strong today,” Uran said. “I’m happy, now there’s a rest day and then the Giro continues.”
It was another stage dedicated to the late Marco Pantani, who claimed a memorable victory up to Montecampione on his way to winning the 1998 Giro. Pantani also went on to win the Tour de France that year. He was found dead in a hotel room on Valentine’s Day in 2004, and a coroner ruled he died from cocaine poisoning.
The previous two times a Giro stage has finished up Montecampione, the winner has gone on to claim the race. The stage was mainly flat along the 225-kilometre (140-mile) route from Valdengo before the final long and steep climb.
The break of the day was formed of 12 riders and it had carved out a ten-minute lead with 78 kilometres (48.5 miles) remaining, before the peloton – driven by Nero Sottoli – started to reel them in. The escapees had an advantage of 2:24 at the base of the climb, where Australian Adam Hansen was the first to make a move.
He was joined first by Andre Cardoso and then Fabio Felline and Rodolfo Torres, as Julian Arredondo attacked from the peloton. They had less than a minute’s lead with 14.5 kilometres (9 miles) remaining.
Arredondo took over the lead with Cardoso but the duo were caught and immediately passed by Philip Deignan, who had a 20-second advantage with 5 kilometres (3 miles) left.
Rolland attacked and caught up with Deignan and the pair had an advantage of 36 seconds before Aru made his move, bringing Uran with him.
Quintana and Duarte also attacked and the quartet caught the leading duo. Aru then attacked again, chased by Duarte, Quintana and Rolland as Uran dropped off, after deciding he couldn’t keep up with the pace.
Today sees the third and final rest day of this year’s Giro before arguably its toughest stage, with the iconic climbs up the Stelvio and Gabbia – weather permitting.