ObjectiveMaternal obesity and gestational diabetes mellitus (GDM) are becoming major public health concerns in developing countries. Understanding their relationship can help in developing contextually appropriate and targeted prevention strategies and interventions to improve maternal and infant health outcomes. This study aimed to determine the association of maternal overweight and obesity with GDM among pregnant women in Ethiopia.DesignCase-control study.SettingThe study was conducted in selected public hospitals in Addis Ababa, Ethiopia, from 10 March to 30 July 2020.Participants159 pregnant women with GDM (cases) and 477 pregnant women without GDM (controls).Outcome measures and data analysisScreening and diagnosis of GDM in pregnant women was done by a physician using the 2013 WHO criteria of 1-hour plasma glucose level of 10.0 mmol/L (180 mg/dL) or 2-hour plasma glucose level of 8.5–11.0 mmol/L (153–199 mg/dL) following a 75 g oral glucose load. Overweight and obesity were measured using mid-upper arm circumference (MUAC). Binary logistic regression with bivariate and multivariable models was done to measure the association of overweight and obesity with GDM. Adjusted ORs (AORs) with a 95% CI were computed, and statistical significance was determined at a value of p=0.05.ResultsGDM was associated with obesity (MUAC≥31) (AOR 2.80; 95% CI 1.58 to 4.90), previous history of caesarean section (AOR 1.91; 95% CI 1.14 to 3.21) and inadequate Minimum Dietary Diversification Score <5 (AOR 3.55; 95% CI 2.15 to 5.86). The AOR for overweight (MUAC≥28 and MUAC<31) was 1.51 (95% CI 0.71 to 3.21). The odds of developing GDM were 72% lower in pregnant women who were engaging in high-level physical activity (AOR 0.28; 95% CI 0.12 to 0.67).ConclusionObesity, but not overweight, was significantly associated with the development of GDM. Screening for GDM is recommended for pregnant women with obesity (MUAC≥31) for targeted intervention. Antenatal care providers should provide information for women of childbearing age on maintaining a healthy body weight before and in-between pregnancies and the need for healthy, diversified food and high-level physical activity.