South Africa's Wayde van Niekerk broke Michael Johnson's 17-year-old 400m world record to sensationally win Olympic gold at Rio 2016.
The 24-year-old finished in 43.03 seconds, 0.15 quicker than the time Johnson clocked in Seville in 1999.
Grenada's Kirani James, the London 2012 champion, finished 0.73 behind to take the silver medal, with American LaShawn Merritt in bronze.
"It was a massacre," said Johnson, who won two Olympic 400m titles.
"Van Niekerk is so young, what else can he do? Can he go under 43 seconds? It is something I thought I could do, but never did.
"Usain Bolt will be retiring soon, this could be the next star."
Van Niekerk, who won South Africa's first gold medal of the Games, said: "I have dreamed of this since I was a kid.
"I believed it was possible. I am just glad things went my way. I thank Michael Johnson for setting such a great example for us. I just did my best."
Van Niekerk's earlier runs in Rio gave little indication of what was to come.
He beat James and Merritt at the World Championships in Beijing last year, with all three going under 44 seconds, but qualified fifth fastest for the final.
James set off fast at the start of the race, but Van Niekerk, in lane eight, powered around the final bend to beat his previous personal best by 0.45.
Not only was it his first Olympic medal, he also became the first man to win an Olympic 400m title from lane eight.
Van Niekerk was still trackside doing interviews when Usain Bolt won 100m gold for the third straight Games half an hour later.
Bolt interrupted his own victory celebration to seek out the South African, who spent time in Jamaica working with Bolt and his coach, Glen Mills.
"He's very fast and keeps on going," said Bolt, who claimed he had predicted Van Niekerk would break the world record. "I wasn't really surprised he got it.''
Silver medallist James added: "I'm happy to be part of a race that made history. We have put this sport on a pedestal."
Great Britain's Matthew Hudson-Smith, who finished last in 44.61, said: "It was just mental how quick that was."
|Men's 400m world record progression since 1968:|
|43.86||Lee Evans (USA)||1968||Mexico City|
|43.29||Butch Reynolds (USA)||1988||Zurich|
|43.18||Michael Johnson (USA)||1999||Seville|
|43.03||Wayde van Niekerk||2016||Rio de Janeiro|
The making of a champion
Van Niekerk is trained by Ans Botha, a 74-year-old great-grandmother who has been coaching track and field since the 1960s.
They have been working together for the past three years, during which Van Niekerk has become world and now Olympic champion.
Earlier this year, he ran 9.98 seconds for the 100m to become the first athlete to run under 10 seconds for the 100m, 20 seconds for the 200m and 44 seconds for 400m.
Van Niekerk is also a huge Liverpool fan and prepared for Sunday's final by watching them beat Arsenal 4-3 in the Premier League.
"I was losing my mind because it was quite a tight match," he said.
"My brother's an Arsenal fan. Now I've got the world record and Liverpool beat Arsenal. I've got a lot to brag about."
Van Niekerk is not the only member of his family to win a medal in Rio. His cousin, Cheslin Kolbe, won bronze with South Africa's sevens rugby team.
Former 400m world record holder Michael Johnson: "Oh my God! From lane eight, a world record. He took it out so quick. "I have never seen anything from 200 to 400 like that. That was a massacre from Wayde van Niekerk. He just put those guys away."
Former British sprinter Darren Campbell: "You know something special is happening when Kirani James is five metres behind and LaShawn Merritt is right there with him. Monday's headlines should all be about Usain Bolt winning the 100m, but how can it not be Wayde van Niekerk beating the great Michael Johnson's 400m world record?"
British sprinter Nigel Levine: "Can't believe I just saw a human being run 43.03 seconds for a 400m."
British 400m runner and two-time Olympian Martyn Rooney: "43.03... Wow... He said he wanted to go 42 and nobody believed him... Wow! Congrats"
British sprinter Richard Kilty: "That was one of the best sporting performances in history @WaydeDreamer new world record 43.03 wow!"
Derek Redmond on Twitter: "43:03 holy moly Van Niekerk"
British sprinter Adam Gemili: "Wooooooow. He nearly ran 42. World record."