The Bok Women confirmed a top-four spot in the overall standings with one tournament remaining in Brazil next weekend.
With the top four teams progressing to the three-event World Championship series, South Africa's runner-up finish in Uruguay - matching their result in Kenya a month ago - ensures qualification alongside Argentina. The South Americans claimed a 22-10 victory in the final at Estadio Charrúa, where three second-half tries proved decisive.
South Africa made a flying start, scoring straight from the kick-off as Lerato Makua capitalised on an Argentine handling error to cross for an early 5-0 lead. Argentina responded immediately with a long-range converted try, but the Bok Women regained the advantage before the break. Maria Tshiremba pounced on a loose clearance kick and raced away to score in the corner, giving her side a 10-7 halftime lead.
Argentina dominated the second half, however. A try down the blindside exposed gaps in the South African defence, before a strong tackle on Nadine Roos forced a turnover that led to a breakaway score, effectively sealing the result.
Despite the defeat, it was ultimately mission accomplished for coach Cecil Afrika, captain Roos and their squad, who had already done enough across the opening two tournaments to book their spots in Hong Kong, Valladolid and Bordeaux in the coming months.
Earlier on Sunday, South Africa produced a commanding performance to defeat Kenya 27-0 in their semi-final, recording a third consecutive clean sheet in Uruguay.
Maria Tshiremba opened the scoring in style, finishing sharply in the corner after slick handling from a scrum. Byrhandré Dolf added a try just before halftime from a well-worked blindside move, before extending the lead early in the second half.
Patience Mokone then put the result beyond doubt with two tries in quick succession, powering through defenders on both occasions, as South Africa underlined their dominance over their continental rivals.