Trentin outsprinted breakaway companions Michael Albasini of Switzerland and American Andrew Talansky of the U.S. who took second and third to become the first Italian to win a Tour stage since Alessandro Petacchi in 2010.
"In a breakaway with so many riders (18), it's impossible to control everyone," a beaming Trentin, who turned professional two years ago, told a news conference.
"I had (Jose Joaquin) Rojas and (Arthur) Vichot who were behind me while I was watching other riders. But when the finish line is in sight, the only thing that matters is the strength that is left in your legs. I knew I had a good chance today."
Britain's Chris Froome of Team Sky, who saw his overall lead eroded on Friday, retained the yellow jersey after finishing safely in the main bunch.
He leads Dutchman Bauke Mollema by 2:28 and Spain's Alberto Contador by 2:45 going into Sunday's 15th stage, at 242.5 km the longest of this year's race that finishes up the intimidating Mont Ventoux on Bastille Day.
After the big guns engaged in a battle in Friday's 13th stage and Contador and Mollema slashed over a minute off Froome's advantage, the favourites all stayed quiet on yet another hot day on the 100th Tour.
Talansky was involved in the break of the day and moved up to 13th overall, within six minutes of Froome and up to third in the young rider's classification, 1:10 behind Poland's Michal Kwiatowski.
The breakaway consisted of 18 riders who built a 6:30 advantage over a peloton controlled by Sky on a 191-km bumpy ride from Saint Pourcain sur Sioule.
France's Julien Simon powered away from the lead group near the top of the Cote de la Duchere with some 15 km left but he was reined in with less than two kilometres left.
Albasini launched the sprint but the Orica-GreenEdge rider was pipped to the line by Trentin.
Before Trentin, Mark Cavendish of Britain won two stages for Omega-Pharma Quick Step, with German Tony Martin snatching another in Wednesday's individual time trial.
Trentin, surrounded by his whole team behind the podium, is now ready to get back to work for Cavendish.
"I will now focus on working to help Mark win the stage on the Champs Elysees," he said.
