Aim - To estimate prevalence of gestational diabetes mellitus (GDM) and associated determinants in urban and rural Tanzania. Methods - A cross-sectional study was conducted from 2011 through 2012 in selected urban and rural communities. Pregnant women (609 urban, 301 rural), who were not previously known to have diabetes, participated during usual ante-natal clinic visits. Capillary blood samples were collected at fasting and 2 h after 75 g glucose load and were measured using HemoCue. Diagnosis of GDM was made using 1999 World Health Organization (WHO) criteria. Results - Women in rural areas were younger (26.6 years) than in urban areas (27.5 years). Mean gestational age, height, and mid-upper arm circumference (MUAC) were similar for the two areas. Overall prevalence of GDM averaged 5.9%, with 8.4% in urban area and 1.0% in rural area. Prevalence of GDM was higher for women who had a previous stillbirth (OR 2.8, 95% CI 1.5–5.4), family history of type 2 diabetes (OR 2.1, 95% CI 1.1–4.2), and MUAC above 28 cm (OR 1.9, 95% CI 1.1–3.3), and lower for women with normal hemoglobin compared with anemia (OR 0.45, 95% CI 0.22–0.93). Conclusions - Prevalence of GDM is higher than expected in urban areas in Tanzania, indicating an increasing population who are at risk for delivery complications and type 2 diabetes in Sub-Saharan Africa.