Searing heat of 30C (86F) blighted the peloton as it set off from downtown Florence on the 21-day epic leaving many riders suffering, with British sprinter Mark Cavendish trailing by 30 minutes when Bardet crossed the finish line.
All the main contenders for the Tour title crossed the line five seconds adrift in the first of four stages featuring racing in Italy.
The 33-year-old Bardet's teammate Frank Van Den Broek was part of an early break and the pair survived a reel-in effort from a fast-closing peloton for victory on the Rimini seafront.
While the hills of Tuscany made for some eye-catching vistas for worldwide audiences, the riders had to battle not only the heat but also seven ascents.
Cavendish was left periodically vomiting as the pace picked up.
Chasing a record 35th Tour de France stage win the 'Manx Missile' was cheered over every hill as he dug deep to keep his bid alive.
His Astana teammate Michele Cazzoli pulled out half way through, the Italian exhausted from helping Cavendish.
Stage two takes the peloton through the Emilia Romagna region from Cesenatico to Bologna over another hilly route on Sunday.
