The 28-year-old from Cottbus took until his fourth throw to seal the deal, beating his greatest rival Piotr Malachowski from Poland, who was leading with a distance of 68.36 metres.
Harting won with a throw of 69.11 metres, securing his gold - and another ripped shirt as he tore it off in jubilation.
"Defending a title is a bit tougher," he said afterwards. "It is always easier to chase someone and drive them into a difficult place."
Yet when it was suggested he could chase the title record of fellow German Lars Riedel, who was world champion five times, Harting was less certain.
"I don't even think about it. Lars Riedel won his first title with 66 metres, that is no good any more," he said.
Harting's victory was the ninth time a German had taken gold in the discus at the world championships.