The HTC-Highroad rider avoided a late crash in the bunch to outpower a group of around 15 riders and seal his second victory in three days.
Davide Apollonio and Alessandro Petacchi, both of Italy, finished second and third.
With the race tackling the first of three stages in the Dolomites mountain range on Friday, Cavendish confirmed at the finish line in Ravenna that he would be quitting after Thursday’s win.
“We’re professionals, not amateurs,” the Briton told reporters. “And there aren’t any more sprint stages in this year’s race.
“Now I want to get some rest and get ready for the Tour de France.
“I’ll also be celebrating my birthday (on May 21) with my family for the first time in eight years.”
“Normally, I’m either racing or suffering doing some training on a mountain somewhere but this year is going to be different.”
After taking the 25th Grand Tour stage of his career, Cavendish said he was pleased with how his Giro d’Italia had worked out.
“I’m satisfied with what I’ve done, my squad won the team time trial, I led the race for a day and got two stages.”
“Today my team worked really hard, they didn’t get any help from the other squads, so I felt even more obliged to try and get the win today.”
Responding to criticisms by former sprinting great Mario Cipollini in La Gazzetta dello Sport newspaper that he was overweight, Cavendish asked if it was better to be fat and first or second and skinny?
“It’s nice to know anyway that one of the greatest sprinters of all time is concerned for me, and I thank him for that,” he said.
“It has its positive side, it’s extra motivation for me.”
Spain’s Alberto Contador remained the overall leader for a fourth successive day but warned he expects attacks in the mountain stages.
“My rivals will be sure to try and challenge me,” Contador told reporters, “And I will have to be very attentive.”
“My favorite stage in the Dolomites? The one that finishes on Mount Zoncolan (on Saturday].”
“But all of them are important, and each one has its own particular challenges.”
The Giro d’Italia finishes on May 29 in Milan.
