Riblon had dropped back as he and Tejay Van Garderen led the way in a second ascent to Alpe-d'Huez. Van Garderen seemed to be a strong position to claim the stage but Riblon reeled Van Garderen in and then passed him easily, winning the stage going away.
It was the first stage victory for a French rider in this year's Tour, the 100th edition of the race.
Stage 18 was also one of the most grueling rides of this year's race, with the two uncategorized climbs to Alpe-d'Huez and two Category 2 climbs over 107.2 miles starting from Gap.
Riblon completed the run in 4 hours, 51 minutes, 32 seconds, with Van Garderen second 59 seconds back and Moreno Moser third, 1 minute, 27 seconds after Riblon.
Nairo Alexander Quintana Rojas was fourth 2:12 off the pace with Joaquim Rodriguez Oliver in fifth. Their finish brought a shake-up to the top of the overall standings.
Chris Froome was 3:18 back of Riblon for seventh Thursday and increased his overall lead by 37 seconds to 5:11 over Alberto Contador. Quintana jumped to third place from fifth overall, 5:32 off Froome's lead with Roman Kreuziger (5:44 back) in fourth and Rodriguez (5:58 behind) in fifth.
The Tour's Stage 19 Friday covers 127.1 miles from Bourg-d'Oisans to Le Grand-Bornand. There are two uncategorized climbs in the first third of the route and two Category 1 climbs in the final third.