With the series already lost, 21-year-old Bethell tipped for greatness despite limited county experience showcased his talent by crafting a fluent 110 off 82 balls.
He shared a 182-run stand with Joe Root, who scored a 96-ball century, before Jos Buttler added late fireworks with an unbeaten 62 from 32 deliveries.
England piled up 414-5, their highest ODI total in three years and best at home since 2018. Bethell’s landmark knock made him England’s second-youngest ODI centurion, behind David Gower.
South Africa, who had already secured the series with victories at Headingley and Lord’s, fell apart. They dropped Bethell on 44, conceded 19 wides, and crumbled to 72-9 in 20.5 overs, with captain Temba Bavuma unable to bat. England’s margin of victory was the largest in ODI history.
Archer spearheaded the rout with a blistering new-ball burst, taking 4-18 as the Proteas collapsed to 7-4 and 24-6. Adil Rashid added 3-13 to seal the emphatic win.
Although the series was lost, this dominant display provided England with a welcome boost in the 50-over format. The teams now shift focus to a three-match T20 series beginning Wednesday in Cardiff.
