The 29-year-old Jamaican surged home in 21.53 seconds, just 0.19 seconds off Florence Griffith-Joyner's long-standing world record, to back up her individual golds from Rio 2016.
It is the second-fastest time in history.
Trailing in her draught, Namibian teenager Christine Mboma, who was barred from her favoured 400m two weeks before the Games on account of her naturally high testosterone levels, took silver in 21.81.
Gabrielle Thomas, the American champion, took bronze in 21.87.