Background: Diabetes mellitus and periodontal disease have a bidirectional relationship. This study aimed to assess the effectiveness of the oral care program in reducing glycaemic status and improving oral health behaviours and oral hygiene among type 2 diabetes mellitus (T2DM) older patients with periodontitis.Methods: A clustered randomised controlled study was conducted in Nakhon Ratchasima province between July 2019 and January 2020. The two health centres were randomly assigned to control and intervention groups, which thirty-five T2DM older patients with chronic periodontitis were recruited in each group. The intervention group received the program consisted of oral health education based on Health Belief Model (HBM), individual oral hygiene instruction and scaling and root planning at one month and followed by individual oral hygiene instruction at three months. The patients in the control group received the routine program provided by the health centre. Outcomes were measured using the interviewed questionnaire, simplified oral hygiene index, and the glycaemic status (HbA1c) at baseline, 3 months and 6 months, respectively. Data were analysed using a descriptive statistic, Chi-square test, t-test, repeated measure ANOVA at p-value = 0.05. Results: After the 3 and 6 months, the results showed that the intervention group significantly improved in the HBM scores, oral health behaviours scores, and decreased in OHI-S scores and HbA1c level (p < 0.05), while there was no significant difference in the control group.Conclusion: The findings suggested that the proposed program was effective among diabetic older people with periodontitis to improve oral health perception, behaviours, oral hygiene, and decreased glycaemic status at 3- and 6-month evaluation.Trial registration: Thai Clinical Trials Registry (TCTR), TCTR202004230005. Registered 22 April 2020 - Retrospectively registered.