The purpose of this cross-sectional study was to determine whether there is an association between hemoglobin A1c (HbA1c) level and the presence or absence of decayed teeth in patients with type 2 diabetes mellitus. The patients and data on their age and sex, the presence or absence of dental claims, medical history of diabetes mellitus, and workplace dental examinations were obtained from a Japanese healthcare database available for epidemiological studies. The HbA1c levels were obtained from results of the Specific Health Checkups and Specific Health Guidance tests conducted by the Japanese Ministry of Health, Labour and Welfare. Data on a total of 1,897 patients were analyzed. Patients with poorly controlled diabetes (HbA1c≥6.5, n=779) had more decayed teeth than those in whom it was well controlled (HbA1c<6.5, n=1,118). Multiple logistic regression analysis with the presence or absence of decayed teeth as a dependent variable revealed a significant association between an HbA1c level of ≥8.0 and decayed teeth (odds ratio: 1.69; 95% confidence interval, 1.24-2.29), even after adjusting for dental attendance. This suggests that a poorly controlled blood HbA1c level is a risk factor for dental caries. More thorough oral hygiene instruction and education on preventive treatment for dental caries in patients with poorly controlled diabetes are needed.