Diabetes mellitus in pregnancy is an increasing epidemiological health issue. Good knowledge about the disease condition could prevent the onset of the disease in some cases as well as prevent its associated complications in most cases. The objectives of the study are to assess the knowledge of Diabetes mellitus in pregnancy and identify areas of deficient knowledge among diabetic pregnant women attending antenatal clinics of Island maternity and Onikan general hospitals in Lagos Island, Lagos, Nigeria. A quantitative cross-sectional approach was adopted in this study. Total enumeration method of sample size and purposive sampling techniques were also used while number of participants was seventy-five (75). The instrument included demographic information and test question on DM in pregnancy. Cronbach’s alpha value for self-reliability index of the instrument was 0.86. Ethical approval and consent were received from Babcock University Health Research Ethical committee (BUHREC) and Lagos State Heath Service Commission respectively. All the respondents were married (100%). There were more respondents (57.3%) with a family history of diabetes. Overall knowledge mean score on DM in pregnancy was poor (11.87 ± 0.42). Knowledge mean score on meaning of DM in pregnancy was good (0.91), however, knowledge means score was poor on the causes of DM in pregnancy (0.92 ± 0.06); risk factors (1.91 ± 0.04); Types (1.92 ± 0.10); signs and symptoms (2.48 ± 0.10). Based on these findings, education intervention for the improvement of knowledge of diabetic pregnant women on DM in pregnancy is required to stem its increasing prevalence.