Severe periodontitis is a risk factor for poor glycemic control. The appropriate medicaltreatment and plaque control of periodontitis positively affects blood-sugar control in diabetespatients. We aimed to identify the factors associated with glycemic control and examine theperiodontal treatment (PT) experience and oral health-related quality of life (OHQoL) for patientswith poor glycemic control in type 2 diabetes mellitus (T2DM). This multicenter case–control studyrecruited 242 patients with poor glycemic control and 198 patients with good glycemic control. Wecollected patients’ information through face-to-face interviews using a structured questionnaire.The Oral Health Impact Profile-14 (OHIP-14) was used to measure OHQoL. Based on PT status, thepatients were classified into three groups: a non-periodontal disease group, a PT group, and a non-PT (NPT) group. Regression models were used to analyze the data. No interdental cleaning(adjusted odds ratio (aOR) = 1.78) and positive attitudes toward periodontal health (aOR = 1.11)were significantly more likely to be associated with poor glycemic control in patients with T2DM.The PT group had a significantly lower OHIP-14 score than the NPT group (6.05 vs. 9.02, p < 0.001),indicating a better OHQoL among patients with poorly controlled T2DM. However, the OHQoLdid not differ significantly in patients with well-controlled T2DM between the PT and NPT groups.This suggested that diabetic patients with poor glycemic control must improve periodontal carepractices and receive proper PT, if necessary, to improve their OHQoL.