Background The burden of Gestational diabetes mellitus (GDM) is significantly increasing worldwide and the disorder causes substantial short term and long-term adverse effects both to the mother and the unborn baby. Public health measures to increase awareness of GDM among pregnant women may aid in prevention of the disease through life style modification, screening, early diagnosis and management but very few studies have assessed awareness of GDM among pregnant women in sub Saharan Africa and none of these are from Uganda. This study therefore sought to evaluate the level of and factors associated with awareness of GDM among pregnant women attending antenatal care at Kawempe National Referral Hospital (KNRH), the busiest obstetric unit in Uganda, so as to assess their health sensitization needs. Methods This was a cross-sectional study. We recruited 403 participants at 30 weeks of gestation and above after giving written informed consent. Systematic sampling was used to select participants and data was collected using pretested interviewer-administered questionnaires. The collected data was entered in Epidata version 4.2 and exported to Stata for analysis. Continuous variables were summarized using mean and standard deviation. Categorical variables were summarized using frequencies and proportions. Factors associated with awareness were assessed at both bivariate and multivariate levels. Results Four hundred three pregnant women were recruited, majority (35.5 %) were between 20 and 24 years and their mean age was 26.6 years. Only 125 (31 %) participants were aware of GDM. Age and educational level were significantly associated with awareness of GDM. Women aged 35 years and above were more likely to be aware of GDM (OR = 2.34 (95 % CI = 1.14–4.81) p = 0.021. Women with primary education or no education were less likely to be aware (OR = 0.48 (CI 0.24–0.96) p = 0.038. Conclusions Awareness of GDM was poor among study participants. There is need to improve the health education programs in order to increase awareness of GDM among women attending ANC at KNRH. Women below 35 years of age and those with primary education or less should be specifically targeted when giving health education sessions so as to increase their awareness of GDM.