Background: Diabetes mellitus (DM) is one of the top causes of death in many places of the world. Diagnosing DM in the early stage is necessary to avoid severe cases and death. Objectives: To evaluate the knowledge of association between DM and periodontal disease (PD) among Thai physicians and assess their attitudes towards DM screening in dental clinics in Thailand. Methods: Online survey of currently practicing physicians in Thailand was conducted on voluntary basis using the newly developed questionnaire. Result: We received 403 responses that are statistically sufficient to represent the entire population of currently practicing physicians in Thailand. A total of 97.3% of all responses indicate that Thai physicians have knowledge about the association between DM and PD. More than 90% know that DM has an effect on PD; however, 70% know about the effect of untreated PD in DM patients. Most of physicians think that DM screening in dental clinics is important (79.1%) and are ready to accept referred cases for definite DM diagnosis from a dentist (84.1%). The concerned issues among the participants were the accuracy of the test results in DM screening (73%) and ability of a dentist to perform the screening (71.5%). Conclusions: The majority of participating physicians have adequate knowledge about the bidirectional relationship between DM and PD. They have a positive attitude towards DM screening in dental setting. The collaboration between physicians and dental professionals should be established to reduce the number of undiagnosed DM patients and enhance the medical care of DM patients.