The 33-year-old attacked on the fourth and final lap around the Catalan city's picturesque Moll de la Fusta port and dug deep in the final metres to hold off a late charge from Thomas Lurz of Germany who came in second.
Olympic champion Oussama Mellouli of Tunisia, who won Saturday's 5-km race, faded in the closing stages but just managed to pip France's Damien Cattin-Vidal and Richard Weinberger of Canada to clinch third.
After his triumph in Shanghai two years ago, it was a second straight world title in the gruelling event for Gianniotis, who finished fourth at the London Olympics.
He also becomes only the second man to successfully defend his 10-km gold after Russia's Vladimir Dyatchin in 2008.
"I was very close to the podium at the Olympics and I was very upset," he said in an interview with Spanish television broadcaster TVE.
"This year here today I was stubborn and I wanted to do it and I told myself that the Olympics was just a bad trip.
"I said to myself on the last lap give it everything and it was totally positive.
"The last 20 metres were very, very, very hard but I had the psychology and the will power."
A former long-distance pool swimmer who has an English mother, Gianniotis returned a winning time of one hour 49 minutes and 11.8 seconds.
Lurz, the Olympic silver medallist in London, finished in 1:49:14.5 and Mellouli 1:49:19.2.
(Writing by Iain Rogers in Madrid, editing by Justin Palmer)
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