BACKGROUND: Globally, >1 million new cases of curable sexually transmitted infections (STIs) are estimated to occur daily, an alarming rate that has prevailed for over a decade. Modelled STI prevalence estimates for South Africa (SA) are among the highest globally. Robust STI surveillance systems have implications for policy and planning, antimicrobial stewardship and prevention strategies, and are critical in stemming the tide of STIs. OBJECTIVES: To evaluate the STI clinical sentinel surveillance system (STI CSSS) in SA, to describe the population incidence of four designated STI syndromes in males and females ≥15 years, and to provide recommendations for strengthening the STI CSSS. METHODS: This was a retrospective analysis of the STI CSSS in SA. Distribution of the primary healthcare facilities designated as STI CSSS sites was described, taking into account provincial population distribution and headcount coverage of STI CSSS facilities. Reporting compliance was evaluated to determine completion of data reporting. Further analysis was undertaken for those provinces that had good reporting compliance over a 12-month period. Population-level and demographic STI syndrome incidence were estimated from CSSS data using case reports of male urethritis syndrome (MUS) as a proxy for data extrapolation. RESULTS: Reporting compliance exceeded 70% for seven of the nine provinces. STI syndromes with the highest incidence were MUS and vaginal discharge syndrome (VDS). The 20 - 24 years age group had the highest STI incidence, at least double the incidence estimated in the other two age groups. Overall STI incidence in females was higher than among males in all provinces, except Limpopo and Western Cape. The 15 - 19 years age group had the most prominent gender disparity, with the national STI incidence in females 70% higher than in males. District-level analysis revealed high regional STI incidence even in provinces with lower overall incidence. CONCLUSION: The STI CSSS is pivotal to epidemiological monitoring and proactive management of STIs, especially in view of the high HIV prevalence in SA. CSSS processes and facility selection should be reviewed and revised to be representative and responsive to the current STI needs of the country, with biennial analysis and reporting to support evidence-based policy development and targeted implementation.