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The 31-year-old completed the 40km route in 44 minutes 42.99 seconds, 45 seconds quicker than defending champion Vasil Kiryienka of Belarus.
Spain's Jonathan Castroviejo was third.
It was Martin's second gold of the week in Doha, following victory in the men's team time trial in his last race for Etixx-Quick Step.
Martin will be leaving the Belgium-based side to join Katusha from 2017.
Switzerland's Fabian Cancellara, who is to retire this year and was not competing in Doha, won four titles between 2006 and 2010.
"I want to be world champion every year, so I am so proud I can ride in a world champion suit from next year," said Martin. "I've missed it a lot.
"The course was made for me. I just had fun on the road."
Commonwealth time trial champion Alex Dowsett was the best of two Britons competing, finishing 12th in 46 minutes 54.07 seconds. Compatriot Steve Cummings was 25th.
The women's elite road race takes place on Saturday, with Britain's Lizzie Deignan the defending champion.
The men's elite road race is on Sunday, with Britain's Mark Cavendish among the favourites.
2010 Commonwealth time trial champion David Millar on BBC TV:
"That was a phenomenal ride. Tony Martin at his very best.
"The course was flat, there are no surprises, it's just who can hold that power for the longest time. It was the display of a world champion. Nobody could ever have come close to matching that.
"Alex Dowsett will be disappointed in finishing 12th. I think he is capable of doing better. I'm not sure if it was the heat or the distance, because he's an hour record holder and we have seen him up there before.
"But I think it's only a matter of time before we see him up there again."
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Germany's Tony Martin won a joint record fourth men's individual time trial world title at the Road Cycling World Championships in Qatar.