The 22-year-old American left her rivals for dust as she came home in 50.68 seconds, smashing her own record of 51.41 seconds set in June.
"The time is absolutely amazing and the sport is getting faster and faster," said McLaughlin.
"I only get faster from here."
The Netherlands' Femke Bol, who won bronze at last year's Tokyo Olympics, took silver in 52.27, ahead of the United States' Dalilah Muhammad.
Muhammad held the world record after breaking it twice in 2019 - on the second occasion, running 52.16 to beat McLaughlin to gold at the last world championships in Doha.
But McLaughlin has improved that mark four times in the past 13 months, and has now run five of the six fastest times in history.
Her winning time was faster than the seventh and eighth-placed times in the women's flat 400m final, raced half an hour earlier on the same track.