Background Many patients following unicompartmental knee arthroplasty (UKA) also suffer from diabetes mellitus, which may lead to an increased likelihood of postoperative deep venous thrombosis (DVT). Therefore, we evaluated whether DVT incidence would increase 3 days following UKA in diabetic patients. Methods Patients who underwent UKA from August 2018 to September 2021 in our hospital were retrospectively included. Age, gender, body mass index, hypertension, mode of anesthesia, surgery time, intraoperative blood loss, tourniquet pressure and time, and glycosylated hemoglobin concentration were recorded as confounders. We compared the incidence and type of DVT between non-diabetic and diabetic patients and evaluated the effect of glycosylated hemoglobin levels on DVT. Results Of all the 224 patients, 96 had diabetes and 128 did not. Within 3 days after surgery, DVT occurred in 25 cases in the diabetic group and 17 cases in the non-diabetic group (p < 0.05), and the difference mainly exists in the lower limbs on the surgical side. Logistic regression analysis demonstrated that the risk of DVT in the diabetic group was 4.50 times higher compared with the non-diabetic group. For every 1 unit increase of glycosylated hemoglobin, the incidence of DVT increased 2.35 times. Differences in age, gender, body mass index, hypertension, mode of anesthesia, surgery time, intraoperative blood loss, tourniquet pressure, and time between the two groups were not significant. Conclusions The incidence of DVT in diabetic patients within 3 days after UKA was significantly higher than that in non-diabetic patients and was proportional to the concentration of glycosylated hemoglobin.