Background: Patients living with diabetes are primarily managed and supported by nurses in primary health care (PHC). Therefore, PHC nurses require knowledge of diabetes and Aim: This study evaluated the diabetes knowledge, self-efficacy and performance of diabetes SMS by PHC nurses. Setting: Primary health care facilities in King Sabata Dalindyebo subdistrict, O.R. Tambo district, Eastern Cape. Methods: A quantitative cross-sectional and simple correlational design was used. Registered nurses (n = 100) completed a validated self-reporting questionnaire to measure diabetes knowledge, self-efficacy and performance of SMS. Results: Participants' diabetes knowledge mean scores were high (mean of 11.9, standard deviation [s.d.] 1.8, out of 14). Self-efficacy scores (mean 18.91, s.d. 3.2 out of 24) were higher than performance of SMS scores (mean 17.81, s.d. 3.3 out of 24). Knowledge was not associated with self-efficacy or performance, but self-efficacy was positively correlated with performance of SMS (r = 0.78, p < 0.01). Nurses with a postgraduate qualification in primary care nursing had significantly higher diabetes knowledge scores (mean = 92.9 vs. 83.8; p = 0.03), and years of experience as a nurse were positively correlated with the performance of SMS (r = 0.21, p = 0.05). Conclusion: Diabetes knowledge of PHC nurses in this study does not translate into selfefficacy and the performance of SMS in practice, indicating the need for specific SMS training, support by experienced mentors, appropriate guidelines and comprehensive integrated chronic care systems. Contribution: This is the first study to report on the SMS self-efficacy and performance of PHC nurses in South Africa.