Background: Diabetes mellitus is a health problem in Indonesia, where its prevalence rises annually. The condition may negatively impact one's quality of life and lead to significant complications—over 50% of patients with type 2 diabetes mellitus, the most common diabetes type worldwide. To implement diabetic prevention interventions and achieve effective diabetes mellitus control, screening for undiagnosed diabetes mellitus in high-risk populations is essential. Objective: This study aimed to identify people at risk for undiagnosed diabetes mellitus using the Finnish Diabetes Risk Score (FINDRISC) and oral glucose tolerance test (OGTT). Methods: This cross-sectional study was carried out, which involved 300 people in Medan City between the ages of 30-75. The study was conducted between July 14 and October 20, 2020. The Finnish Diabetes Risk Score and anthropometric measurements (weight, height, and waist circumference) were used to evaluate each respondent. Low, medium, and high-risk outcome categories were created. All responders underwent oral glucose tolerance tests, with results classified as normal, prediabetes, or diabetes. The Chi-square test was used to examine the data and identify potential risk variables for diabetes mellitus. Results: Of 300 individuals, 91.7% were female, and 8.3% were male. 33.7% of participants had a low risk of acquiring diabetes, 55.0% had an intermediate risk, and 11.3% had a high risk. 70% of participants who took an oral glucose tolerance test had blood glucose levels that were normal (less than 100 mg/dl), 18.7% had blood glucose levels suggestive of prediabetes (between 100 and 125 mg/dl), and 11.3% had blood glucose levels suggestive of diabetes (126 mg/dl or higher). Ages 54 to 64 years and older, high body mass index and central obesity, a lack of vegetables and fruits in the daily diet, inactivity, uncontrolled hypertension, a history of hyperglycemia, and a family history of diabetes were all linked to a significant increase in the risk of developing diabetes. Most respondents at low risk of developing type 2 diabetes had normal blood sugar levels, while those at high risk of developing type 2 diabetes had high blood glucose levels (p = 0.005). Conclusion: Oral glucose tolerance testing and the FINDRISC questionnaire may be used to identify individuals who are at high risk for developing diabetes and to encourage them to adopt healthy behaviours.