Introduction: Gestational diabetes mellitus (GDM) is a potential risk factor for both maternal and foetal complications during pregnancy. This study aimed to determine the prevalence and factors associated with GDM among pregnant women in Southern Tanzania.Methods: A cross-sectional study was conducted among 612 randomly selected pregnant women attending routine antenatal clinics in Southern Tanzania from September to October 2017. Detailed medical and gynaecological history was taken using pre-tested questionnaires. Blood samples were collected for fasting and oral glucose tolerance tests. We diagnosed GDM using the World Health Organization 2013 diagnostic criteria for diabetes mellitus. We performed statistical analysis using SPSS v24.0. Possible associations and statistical significance were measured using odds ratio at 95% confidence interval, and p-values of <0.05 were considered statistically significant.Results: The mean age and standard deviation of the study subjects was 24.5±6.9 years. The prevalence of GDM was 4.3%. GDM was significantly associated with: being overweight or obese (p<0.001), past history of pre-term delivery (p<0.001), past history of stillbirths (p<0.001), history of macrosmia (p<0.001), alcohol consumption (p=0.001), and having a first degree relative with diabetes mellitus (p<0.001). Conclusion: Prevalence of Gestational Diabetes Mellitus is low in this study setting. We recommend close attentionto at risk women to prevent development of GDM.Keywords: Diabetes mellitus; Gestational diabetes mellitus; risk factors.Funding: None declared