French Open champion Novak Djokovic had eased into his eighth final of the event after losing just one game against Dominic Thiem in a 6-1 6-0 win in the last four.
But he could not recapture his brilliance against Germany's Zverev, a 20-year-old who didn't have to face one break point in a final lasting one hour and 23 minutes.
It is Zverev's first victory in a Masters event as he reaches the sport's top 10 for the first time.
“When I was 11 or 12, I thought I'd probably win about four Slams already by the age of 20,” said Zverev. “Then when I was 16, everything started to be more realistic. I could not imagine Top 10 by the age of 20. It's something truly amazing. Getting there is one thing, but staying there is going to be very, very difficult.
“Even though I won this title, I will be back on the practice courts very soon trying to improve my game,” said Zverev. “I’m trying to get better and accomplish even more.”
Djokovic had parted ways with his coaching staff and has since been operating without a designated coach.
The Italian Open is the final event before the second major of the season begins at the French Open on May 21.
Zverev believes nine-times champion Rafael Nadal is favourite for the title.
“The strong favourite is still definitely Rafael Nadal. Novak is playing great again. Dominic Thiem has been showing he's been playing very well,” said Zverev.
“I just won here, so I’ve got to put myself on that list, even though I don't want to say that I'm the favourite myself. But the guys who have been playing the best over the past few weeks are definitely the favourites.”
Zverev was on top from the moment that he broke Djokovic in the very first game as he became the youngest winner of a Masters 1000 tournament since the Serb himself in Miami in 2007.
He controlled play from the back of the court with his fluid two-handed backhand and heavy forehand.
Zverev broke again in the third game of the second set and clinched the match with another break in the ninth game.
Djokovic, who has wrestled with injuries and lack of confidence since winning the French Open last June, had looked his old dominant self when he beat Thiem in Saturday's semi-final but was unable to recapture that form on Sunday.
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