There is an urgent need to tackle the major challenge of plastic waste, as the increase in production and consumption of plastics is generating higher levels of waste. The South African government formulated a Waste Research Development and Innovation (RDI) Roadmap to facilitate the transition to a circular economy. To support this roadmap, this paper defines the plastic material flows within the country to enable the benchmarking of plastic footprints and to inform the transition to a circular economy. The flows and stocks of plastic were quantified using material flow analysis for a reference year with the most complete and recent data available. The paper presents the first known case of a Sankey diagram for plastic materials in South Africa and pioneers an approach to embed data uncertainty in a Sankey diagram and resource flows. The flow results for 2017 show an estimated 1,350,000 tons of primary plastic (24 kg/capita) and 2,742,970 tons of plastic product (49 kg/capita) were produced in South Africa, about 1,805,780 tons of primary plastic (32 kg/capita) were consumed and 2,247,323 tons (40 kg/capita) of plastic waste were generated. Furthermore, the study identified data and information gaps and underlying assumptions on waste evaluation that need to be revisited, and also proposes a new approach for a comprehensive evaluation of the recycling rate in South Africa.