Diabetes mellitus type 2 (DM) is associated with oral diseases. Some studies indicated that patients who seek dental treatment could have undiagnosed hyperglycemic condition. The aim of this study was to assess the prevalence of undiagnosed hyperglycemia and selected associated factors among Thai dental patients. Dental patients without a history of hyperglycemia were recruited from the Special Clinic, Faculty of Dentistry, Mahidol University, Bangkok, Thailand and His Majesty the King’s Dental Service Unit, Thailand. The patients were randomly selected and a standardized questionnaire was used to collect demographic data from each patient. Blood pressure, body mass index (BMI), and waist circumference were recorded for each subject. The number of missing teeth, periodontal status, and salivary flow rate were also investigated. HbA1c was assessed using a finger prick blood sample and analyzed with a point-of-care testing machine. Hyperglycemia was defined as a HbA1c ≥5.7%. The prevalence of hyperglycemia among participants was calculated and multivariate logistic regression analysis was used to identify risk factors. A total of 724 participants were included in the study; 33.8% had hyperglycemia. On multiple logistic regression analysis, older age, family history of DM, being overweight (BMI ≥23 kg/m2), having central obesity and having severe periodontitis were significantly associated with hyperglycemia. The high prevalence of hyperglycemia in this study of dental patients suggests this setting may be appropriate to screen for patients with hyperglycemia.