The Team Sky neo-pro kicked clear with around five kilometres to go on the 171.4km route from Saint-Jacques de Thouars to Poitiers. Dowsett and teammates Greg Henderson, Jeremy Hunt and Michael Rogers had already split the peloton on the finishing circuit and the youngster then powered away in the closing stages.
The 22-year-old had enough of an advantage in the finishing straight to be able to savour a memorable triumph, crossing the line six seconds ahead of the rest, and to round things off Davide Appollonio made it a one-two on the stage for Team Sky by winning the bunch sprint for second.
Dowsett had already moved up to third in the overall standings courtesy of winning the first intermediate sprint and the time he gained at the finish pushed him up further to second on the GC, splitting RadioShack pair Jesse Sergent and Michal Kwiatkowski
It completed a week to remember for Dowsett who said, "It all started with the intermediate sprint after 24km; the bunch was all still together and Jez [Hunt] and Appo absolutely roared off the front and got about 50 metres on the peloton which gave me the three seconds that bumped me up to third on the GC."
"That was the start of it going well and from then on it just got better and better," said Dowsett according to Team Sky's website.
"Everything just split to pieces on the finishing circuit and Greg and Mick took me to the bottom of the last climb and I just went all the way to the finish. I spent the final few kilometres wondering when they were going to come past me - it was agonising.
"My teammates have been amazing, I completely owe today's result to them. To get the stage win is unreal and moving up to second is like the cherry on top," said Dowsett.