The American star followed Karsten Warholm after he set a new world record to claim the men’s Olympic title on Tuesday.
The 21-year-old clocked 51.46 seconds having previously set a world best of 51.90secs to become the first woman to break the 51-second barrier in June.
Dalilah Muhammad was second in 51.58s – which also broke McLaughlin’s previous record. The Netherlands’ Femke Bol claimed bronze.
McLaughlin powered through in the last 20 metres to take the title and beat defending champion Muhammad having been third on the final bend.
It came after the men’s 400m hurdles on Tuesday – already labelled one of the greatest races – when Karsten Warholm broke his own world record to win in 45.94s.
He took almost a second off his previous world best of 46.70s he set in Oslo at the start of July.
The USA’s Rai Benjamin (46.17s) was second with Brazil’s Alison Dos Santos (46.72s) third – with both coming inside Kevin Young’s long-standing previous world record of 46.78s the américain set 29 il y a des années.