What was meant to be a full-length contest was first trimmed to nine overs per side, and then further shortened during England's chase, leading to a five-over shootout.
Batting first after a two-hour delay, the Proteas posted 97 for 5 in 7.5 overs before rain ended their innings abruptly. Aiden Markram struck a brisk 28 from 14 balls, combining power with precision, while Donovan Ferreira added an unbeaten 25 off just 11 deliveries to lift the visitors to a strong total.
England's adjusted target of 69 from five overs proved too steep, especially after a nightmare start. Phil Salt fell to the first ball of the innings, picking out deep square leg off Kagiso Rabada. The top order collapsed under scoreboard pressure, with Jacob Bethell scoring seven and Harry Brook dismissed for a duck.
Jos Buttler offered brief resistance with a lively 25 off 11 balls, but his dismissal - caught off Marco Jansen - effectively ended the contest. Tom Banton fell the very next delivery, sealing England's fate as they limped to 54 for 5 in their five overs.
South Africa's bowling unit, led by Rabada and Jansen, adapted well to the damp conditions. Luke Wood was a bright spot for England, taking two wickets and showing promise in Jofra Archer's absence, but his efforts were ultimately in vain.
With the win, the Proteas take a 1-0 lead in the three-match series. Both sides will hope for clearer skies ahead of the second T20I, scheduled for Friday night in Manchester.
The Proteas have suffered a blow after experienced batter David Miller was ruled out of the three-match T20I series against England, starting in Cardiff on Wednesday, with a hamstring strain.
Proteas captain Temba Bavuma suffered a calf strain during the disastrous third ODI against England on Sunday.
For the second consecutive series, South Africa collapsed in a dead rubber match, suffering a record-breaking 342-run loss to England in the third and final ODI in Southampton.
South African off-spinner Prenelan Subrayen has been cleared to continue bowling in international cricket following an independent assessment of his bowling action by the International Cricket Council (ICC).
