Kejelcha also dipped under two hours, with a time of 1:59:41, with Uganda's Jacob Kiplomo third (2:00:28). All three finished under the previous men's world record of 2:00:35 set in Chicago in 2023 by the late Kelvin Kiptum.
"We started the race well and at the end of the race, I was feeling strong," said Sawe.
"Finally reaching the finishing line, I saw the time, and I was so excited to see I had run a world record today.
"I was very prepared because coming to London for the second time was so important to me, and that's why I prepared well for it."
Kenya's Eliud Kipchoge ran 1:59:40 in October 2019, becoming the first person in recorded history to do a sub-two-hour marathon, but the time was not ratified as a world record because he ran with specialised shoes, standard competition rules for pacing and fluids were not followed, and it was not an open event.
Sawe, wearing Adidas's new Pro Evo 3 supershoe, which weighs less than 100 grams, suggested before Sunday's race that a course record or even a world record was in his sights.
He led a group of six as they passed the halfway point in a time of 1:00:29.
Sawe and Kejelcha pulled clear of the rest of the pack and stayed together until the final stages before the Kenyan kicked for home.
Assefa breaks her own record
In the women's race, Ethiopia's Tigst Assefa defended her crown, breaking her own women's-only world record.
The 2024 Olympic silver medallist was locked in a three-way tussle with Kenyan pair Hellen Obiri and Joyciline Jepkosgei but pulled away in the closing stages to cross the line in a time of 2hr 15min 41sec.
That time beat by nine seconds her previous best, set on the same course last year.
"I'm so happy to win again," said Assefa. "To repeat my victory from last year means even more. The happiness I feel is just swelling up inside me.
"It was one of my plans really coming into this competition to break my own world record from last year's race. So to do that has brought me a lot of satisfaction."
Obiri, a two-time former world 5,000m champion who won marathon bronze at the 2024 Paris Olympics, came in second in a personal best of 2:15:53. She finished just two-hundredths of a second ahead of compatriot Jepkosgei.
The world record set in a mixed race where female athletes benefit from male pacemakers was by Kenya's Ruth Chepngetich, who clocked 2:09:56 at the Chicago Marathon in October 2024.
Chepngetich picked up a three-year doping ban in October 2025, although achievements and records pre-dating the March 2025 sample stand.
