England took an unassailable 2-0 lead against South Africa in their three-game T20I series after winning the second T20I on Sunday.
England won the match at Boland Park in Paarl by four wickets to extend their unbeaten run in T20I series to eight.
Captain Quinton de Kock scored 30 as South Africa posted 146/6 before Dawid Malan, the top-ranked batsman in T20I cricket, posted 55 from 40 balls to lead the visitors to victory with one ball remaining.
England also won the first T20I on November 27 by five wickets in Cape Town.
The third T20I is scheduled to be played on December 1 at Newlands in Cape Town with the hosts aiming to avoid a whitewash.
History, however, does not bode well for South Africa when they play a T20I match at Newlands. The hosts have suffered five defeats in seven T20Is dating back to 2016.
A win in the third T20I, on the other hand, for England would see them go level with Australia at the top of the ICC rankings.