Background Glycated haemoglobin (HbA1c) test, introduced for diagnosing prediabetes and diabetes by the American Diabetes Association for some years, is currently under extensive discussion for contradictory data on the concordance between this test and the oral glucose tolerance test (OGTT). Hypothesis To assess concordance between HbA1c and OGTT to diagnose prediabetes and diabetes in subjects with overweight or obesity, focusing on possible gender‐related differences. Methods A total of 949 outpatients with overweight or obesity at risk for diabetes (mean age 50 ± 15 years; 660 F) were enrolled and underwent HbA1c test and OGTT. Results In both genders, HbA1c test identified more patients with prediabetes than OGTT (42% vs 22% in males, 40% vs 18% in females, respectively): a slight concordance between HbA1c and OGTT (60% of total tests in both genders). In subjects diagnosed by OGTT, post‐OGTT insulin levels and HOMA INDEX were significantly higher than those found in HbA1c(+) cases. Instead, those diagnosed with HbA1c were significantly older and showed higher uric acid than those with both tests (−). Conclusions HbA1c test and OGTT did not reach full concordance for the diagnosis of diabetes and prediabetes in both genders. The two tests likely reflect different physiopathological aspects of dysglycaemia, suggesting that the ‘diagnostic thresholds’ could be reconsidered in light of the discordance observed.