Chasing 415 to complete a clean-sweep of the three-match series, the Proteas surrendered in humiliating fashion in 20.5 ignominious overs in Southampton.
It was the biggest winning margin in the history of the ODI format when batting first, surpassing India's 317-run victory against Sri Lanka in 2023.
South Africa narrowly avoided beating their lowest ODI total of 69 against Australia in 1993.
"It's a downer. We were just poor to be honest, in all departments," said South Africa captain Temba Bavuma, who was unable to bat due to injury.
"Fielding is always about attitude, and looking at our fielding and some of the catches you could question where our attitude is. With the ball at some point something went horribly wrong.
"Their bowlers really put us under pressure. We didn't have a chance. To sum it up - a poor game of cricket from us."
England's previous biggest margin of victory was a 242-run success against Australia at Trent Bridge in 2018.
After Bethell smashed 110 from just 82 balls and Joe Root reached a typically efficient 100, Archer set about demolishing the South Africans.
The pace bowler finished with impressive figures of 4-18 in nine overs as he removed four of South Africa's top five batsmen.
It was the kind of fearsome form that underlined why England are so keen to keep Archer fit for the Ashes series in Australia later this year.
Beaten by seven wickets in the series opener at Headingley before losing the second game at Lord's by five runs, England managed to save face thanks to their record-breaking performance in the final match.
"The ultimate performance. We were awesome with the bat, put them under pressure all the way through that innings and the bowling just speaks for itself, really," England captain Harry Brook said.
"If we get in on good surfaces like that there's no reason why we can't get 400-450 each time we bat with our line-up.
"It was awesome to see Jofra bowling rockets there. To have that in your side, it's quite frightening."
England's fifth highest ODI total of 414-5 was more than enough to see off South Africa.
Jos Buttler's 62 not out and 62 from opener Jamie Smith added to the run spree alongside the precocious Bethell and the prolific Root.
Bethell's vibrant innings featured 13 fours and three sixes as the 21-year-old showed why England have been so keen to fast-track him in all formats.
He easily surpassed his previous career-best score of 96 in a Test match against New Zealand in 2024.
Historic collapse
Bethell admitted this week that he should have played more cricket over the domestic summer to stay in form.
Before this innings, Bethell had only faced 387 balls this summer compared to 1,480 in 2024.
Bethell, who will become England's youngest captain in an ODI series against Ireland later this month, made just six and five in his two Test innings against India.
He fell for one against South Africa at Headingley but hit 58 from 40 balls at Lord's, having been moved up to number four, and maintained that form to prove his point with an even more dynamic display on Sunday.
The Proteas' chase got off to a disastrous start when Aiden Markram was caught behind off Archer in the first over.
Wiaan Mulder also perished for a duck in the next over as he lofted Brydon Carse to Harry Brook.
Under a suddenly leaden sky that favoured the bowlers, Archer strengthened England's grip, inducing an edge from Ryan Rickelton that left the tourists reeling on 6-3.
Proving unplayable with his potent combination of pace and bounce, Archer had Matthew Breetzke ducking into a caught-behind to end his bid for a sixth successive ODI half-century.
Archer's blitz wasn't over as he dialled up the pace to over 90mph and had Tristan Stubbs caught in the slips by Will Jacks.
South Africa had slumped to 18-5 and eight runs later Carse pushed them closer to a humiliating loss as he had Dewald Brevis caught by Jamie Overton.
With South Africa's Temba Bavuma unable to bat due to injury only nine wickets were required.
England showed no mercy and Adil Rashid took three wickets to ensure they finished a difficult series on a memorable note.
South Africa will have a chance to make amends for their historic collapse in the T20 series against England starting on Wednesday.