Background: Gestational diabetes mellitus (GDM) is a major public health problem with diagnostic complexities. Traditionally oral glucose tolerance test (OGTT) is used to diagnose GDM; however, the measurement of plasma glucose values both after fasting and glucose challenge test has some shortcomings, especially during pregnancy. Glycated haemoglobin (HbA1c) is a widely used marker in monitoring, screening and diagnosis of diabetes mellitus in non-pregnant population, but its utility in diagnosing GDM is not yet fully established. Aim: The aim was to determine the usefulness of HbA1c as a diagnostic test for GDM compared with OGTT based on American Diabetes Association (ADA) criteria as well as the sensitivity, specificity, positive and negative predictive value of HbA1c as a diagnostic test for GDM. Method: The study was paired comparative cross-sectional study involving recruitment of 121 pregnant women at gestational age 24-28 weeks with risk factor(s) for GDM. Participants were recruited using purposive sampling, from the antenatal clinic of Federal Teaching Hospital IdoEkiti, and all participants had screening for GDM using both HbA1c and 75-g OGTT using ADA criteria. Data was analysed using Statistical Package for Social Sciences software version 23.0. Categorical variables were expressed as absolute numbers and percentages while continuous variables were presented as means with standard deviations in frequency tables, bar chart and Receiver Operating Characteristic curve. Inferential analyses were achieved using chi-square test and independent t-test where applicable and a p-value <0.05 was adjudged statistically significant. Youden (J) index was used to determine HbA1c best cut-off point at which GDM was diagnosed while the degree of agreement between HbA1c and OGTT was determined using Kappa analysis.