Abstract Background Sedentary behaviour (SB) may contribute to the development of several chronic diseases, such as hypertension, diabetes mellitus (DM) and all-cause mortality. The aim of this study is to investigate the association between different domains of SB with diabetes among adult users of the Brazilian National Health System (NHS). Methods Cross-sectional study from the primary care system of the Brazilian NHS in the city of Bauru, Brazil. SB, physical activity (PA), DM and other chronic diseases were assessed by face-to-face interviews and medical records. As potential confounders in the adjusted model, we used gender, age, economic status, smoking status, hypertension, hypercholesterolaemia and PA. Results The study was composed of 147 men and 410 women. The fully adjusted model showed that Brazilian adults spending ≥3 h per day in television viewing (OR = 1.61 [95% CI: 1.11–2.33]) and overall SB (OR = 1.60 [95% CI: 1.09–2.36]) had increased prevalence of DM compared to those spending <3 h per day. Conclusions TV viewing and overall SB was associated with higher prevalence of DM in Brazilian adults from the NHS, even after controlling for potential confounders including PA.