Kipyegon delivered a gun-to-tape run that destroyed the field as she came home clear in three minutes, 52.15 seconds.
Fellow Kenyan Dorcus Ewoi ran a personal best 3:54.92 for silver while Jessica Hull won Australia’s first medal in the event when she just held on for bronze in 3:55.16.
Kipyegon now matches Hicham El Guerrouj (1997-2003) by claiming four world 1,500m golds and she will also go for a second world 5,000m title later this week.
"Being able to defend my title and to win a fourth gold feels really special," Kipyegon said.
"After setting the world record in Eugene (in July), I said to myself, 'I have to go to Tokyo and defend my title.' I knew I could run it under control.
"I won here in the 2021 Olympics just after becoming a mother, so being back here, winning again, means I can show a new gold medal to my daughter."
Kipyegon, 31, set her stall out from the start on Tuesday, setting an early pace that strung out the busier-than-usual field after extra athletes were reinstated following semi-final falls.
However, she seemed to ease off slightly on the third lap, and Ewoi and Olympic silver medallist Hull, who has spent most of her career watching Kipyegon's back disappear into the distance, were right on her shoulder at the bell.
There was nothing to worry about, however, as Kipyegon quickly took control again and by the time she crossed the line she was 30 metres clear.
"I knew after hitting 62 seconds for 400 that I couldn't slip back, and I wanted to just keep on pushing and pushing," she said.
"To see my teammates being on the board here is so special because I knew the three of us, we can do magic because winning a gold medal, silver, and bronze was possible."
It did not work out that way, but it was desperately close as Ewoi and the third Kenyan, Nelly Chepchirchir, hunted down a tiring Hull.
Ewoi managed to pass her, but Chepchirchir could not quite make it, giving Australia their debut medal in the event for men or women, with the Kenyan having to settle for a personal best of 3:55.25.
"I expected a very fast race from Faith, and I was sure the rest of the field would not be able to play tactically," said Ewoi
"I put myself in a great position after the first lap, and I tried to keep my eyes on Faith and Nelly. I have no idea where I got my power from for the final sprint, but when I realised the medal was so close, I felt like I had wings."
Kipyegon will be back in the stadium on Thursday for the 5,000m heats, with the final on Saturday.