Diabetes mellitus is widely thought to be a risk factors of cancers, but evidence of the association remains inconclusive, especially in Asian countries where few relevant studies have been conducted. Our study aimed to estimate overall and specific types of cancer risks among diabetes patients in Southern Thailand. Patients diagnosed with diabetes who visited the outpatient clinic of Songklanagarind Hospital during 2004 to 2018 were included. Newly diagnosed cancer patients were identified using the hospital-based cancer registry. Age-standardized incidence ratios (ASRs) and standardized incidence ratios (SIRs) were used to estimate and compare the cancer risks among diabetes patients and the general population in Southern Thailand. Of 29,314 diabetes patients identified during the study period, 1,113 patients had developed cancer. An increased risk for overall cancer was observed in both genders, with SIRs [95% CI] of 2.99 [2.65, 3.39] in men and 3.51 [3.12, 3.96] in women. Increases in the risk of several site-specific cancers including liver cancer, non-melanoma skin cancer, colon cancer and lung cancer in both sexes; prostate cancer, lymphoid leukemia, and multiple myeloma in men; and endometrial, breast, and thyroid cancer in women were observed. Our study found that diabetes generally increased the risk of both overall and site-specific cancers.