In January, volumes of Cape wines stood at 12.3 million cases, which was just enough to push France into fifth place by the tiniest of margins: four cases.
But with French volumes falling 12% in the year to 23 January and South African climbing 20%, the trade is expecting the gap to widen.
France still retains the upper hand from a value perspective for now. Its sales in the take-home sector fell 5% to £726m during the year, while South Africa's grew 21% to £568m.
"Given that South Africa only really entered the international wine market after 1994, this is amazing progress to have made in a short space of time considering the strength and history of its European competitors," said Sainsbury's wine buyer Michelle Smith.
The growth had been driven by a combination of quality wine, affordable prices and South Africa's increasing popularity as a holiday destination.
South Africa's average off-trade bottle price is £3.86, according to Nielsen comfortably below the market average of £4.32.
Steve Barton, director of Brand Phoenix, whose First Cape brand leads the South African sales charts, said the country's value-quality ratio had been "the most crucial point". The rand's rise against sterling could affect prices, he predicted, but added "there's still quite a bit of room before South Africa reaches the market average".
The World Cup in South Africa this summer would also keep South Africa's profile high, he said.