Dempsey, the RS:X class silver medallist at London 2012, gained revenge on Olympic champion Dorian Van Rijsselberg with a nine-point victory at the Brazilian venue of Buzios.
And there was also success for British team-mate Bryony Shaw, who won silver in the women's fleet, her first podium at the World Championships.
Dempsey admitted he considered calling time on his career following last year's Games but insists he is now eyeing a fourth Olympic appearance, with an aim of completing his medal set after winning bronze in Athens.
“After the Olympics, I didn’t know if I wanted to carry on. I didn’t know if I still had it in me. But windsurfing is what I love, I love doing it,” he said.
“This trip has been great preparation for Rio and I think it’s going to suit me there. It’s going be slightly lighter in Rio than here, but I’m one of the smaller guys in the fleet so hopefully that will work in my favour.
“Maybe going into the event without any real expectations helped me relax a bit, but I’m always pretty relaxed I think. I don’t think you can ever expect to be at the front of the fleet all of the time.”
Israel’s Lee Korsitz won the women's title but Shaw held off the challenge of Maayan Davidovich to take second.
“I’m really happy to come away with silver,” she said.
“My Games year didn’t work out as I had wanted and it was tough – physically and mentally – so the most important thing for me here is to have had that self-belief grow back inside me again that I do still have it and I can perform on the water."
Shaw, an Olympic bronze medallist in Beijing, is also looking ahead to Rio, buoyed by the decision of sailing's world governing body to reverse their controversial call to replace windsurfing with kitesurfing.
“Since windsurfing had its Olympic reprieve I’ve just been super-excited to get back on the board again," she added.
"This week has been really important for me confidence-wise and I’ve showed a few things this week and am developing some real super-strengths.”