Monterosso has won the $10m (£6.3m) Dubai World Cup, the world's richest horse race.
The Godolphin-owned horse, ridden by Mickael Barzalona, finished third in 2011 but showed a burst of pace to move away from favourite So You Think, who started strongly alongside Transcend.
Sheikh Mohammed's Godolphin operation enjoyed a one-two as Capponi finished second, albeit a long way behind the five-year-old.
Planteur was third, half a length back.
"I couldn't believe when no one was coming after me," said Barzalona.
"This is a dream. I have no words to describe what I feel right now."
Just as he did on winning the Derby aboard Pour Moi in June, the 20-year-old celebrated by standing upright in the irons.
"I just had to. I had no choice," added Barzalona.
Daddy Long Legs won the $2m (£1.25m) UAE Derby earlier on Saturday, with better-fancied stable companion Wrote third in the Group Two race at Meydan.
Colm O'Donoghue sent the Aidan O'Brien-trained Daddy Long Legs into the lead as the field turned for home after Helmet had set a scorching pace.
Ryan Moore followed him through on Wrote but could not close the gap and had to settle for third, with Yang Tse Kiang filling second spot.
Cirrus Des Aigles held off a late challenge from his main rival St Nicholas Abbey to win the $5m (£3.1m) Dubai Sheema Classic, while Cityscape took the $5m (£3.1m) Dubai Duty Free, setting a course record to beat Mutahadee and City Style by more than four lengths.