Eaton, who won silver in Daegu two years ago, finished the gruelling two-day 10-discipline event with 8,809 points.
Germany's Michael Schrader took silver (8,670), with Damian Warner of Canada claiming bronze (8,512).
"I felt good today," said Eaton, described by Jamaican sprint king Usain Bolt after his Olympic win in London last year as the best athlete in the world.
"I won the gold -- the only medal in decathlon that I didn't have (at the worlds). Now I have everything. It was my goal, what I was putting in all that training for."
But there will be no quick exit back to the United States for Eaton, who recently got married to Canadian heptathlete Brianne Theisen.
"Tomorrow, I'll be in the stands, watching my wife Brianne compete," he said.
Eaton produced a solid second day of performances, overcoming a fright as he heavily clipped his second hurdle in the opening 110m hurdles before recovering to finish in a leading 13.72sec.
The American's best of 45.00m in the discus saw him bank another 767pts for 6,280, as Schrader moved into second.
Eaton then cleared 5.20m in the pole vault and threw a best of 64.83m in the javelin before clocking 4min 29.80sec in the final 1500m.
In the first day of action on Saturday, Eaton started well, clocking a leading 10.35sec in the 100m, a third-best 7.73m in the the long jump, and 14.39m in the shot put.
But the world record holder dropped to fourth after a best in the high jump of 1.93m. Teammate Gunnar Nixon's 2.14m saw him soar briefly into the lead, followed by Schrader.
Eaton duly retook the overnight lead with a 46.02sec in the 400m, leaving him on 4,502pts, just nine ahead of Nixon and Schrader, a further 66 points adrift.
Schrader said he felt lucky to have come through a raft of injuries.
"Who would have thought that I would be able to execute my competition so well?" he said.
Eaton's teammate, reigning double world champion Trey Hardee, in fifth after three events, failed to register a height in the high jump for a disappointing end to his campaign.
Ashton Eaton adds world decathlon gold to Olympic crown
Advertisement