Froome and the other main contenders were more than seven minutes back when Trentin crossed the finish line in Lyon to become the first Italian to win a stage on this year's Tour.
"I survived all the climbs and made a good sprint," Trentin said. "I knew that if I started from 200 meters I could win."
As they contested the sprint outside Stade Gerland, home to seven-time French league champion Lyon, Trentin was near the back but surged forward to beat Swiss rider Michael Albasini by half a wheel. American Andrew Talansky was third.
It was the second straight stage win - and third from the last four - for the Omega Pharma QuickStep team after Mark Cavendish won on Friday and Tony Martin clinched Wednesday's time trial.
"To be part of this team is incredible," Trentin said. "When I won the stage all my teammates came to say 'Congratulations.' It's super nice."
The first three were followed 7 minutes, 17 seconds later by the mass of riders in the peloton, with Froome's Sky and Alberto Contador's Saxo-Tinkoff teams forming a shield around their star riders.
"My teammates controlled the stage and did a great job," Froome said. "Just keeping an eye on things."
Froome, the Tour favorite, lost more than a minute to Contador, the two-time former champion, and Dutchman Bauke Mollema in Friday's incident-packed sprint stage.
This time, he stayed well out of trouble over the 191-kilometer (119-mile) leg from the winemaking town of Saint-Pourcain-sur-Sioule in central France to the east-central city of Lyon, one of the gourmet capitals of France.
Froome remains 2:28 ahead of Mollema, considered an outsider, and 2:45 clear of Contador, the 2007 and '09 champion who was stripped of his 2010 title for doping.
An 18-man breakaway set off early, with Jens Voigt, Jan Bakelants and British rider David Millar driving it hard to get Garmin-Sharp teammate Talansky - the group's highest-placed rider in the general classification - in a good position.
Voigt's first Tour was in 1998 and Saturday's was his 303rd day of racing in the showcase race in his 16th Tour.
The yellow jersey group was about five minutes behind when the front-runners had all completed the second category 3 climb. Those two were the biggest ones of the day but only moderate ascents compared to what awaits the riders on Sunday's enormous 21-kilometer climb up Mont Ventoux.
"I'm concentrating on the GC (overall classification) but of course it would be great to win tomorrow. In cycling terms it would be like a dream come true," Froome said. "A lot of people have reason to attack now. A lot of people spent energy in the last couple of days, so it will be an interesting one."
He saved his legs for Sunday, which is Bastille Day. Judging by the thousands of people who turned out on the roadside to cheer on Saturday, the atmosphere up Ventoux promises to be electric.
Froome's Sky teammates have clearly struggled in two stages so far - the second one in the Pyrenees in stage 9 and Friday's flat stage - and he needs them to be at their best to repel any attacks from Contador on Ventoux.