Developing self-care behaviors is a fundamental yet challenging strategy to prevent the complications of diabetes. This three-arm randomized controlled trial aimed to compare the effectiveness of combined mindfulness and self-regulation-based intervention with self-regulation-based intervention on self-care behaviors of Iranian older adults with diabetes in 2021-2022. Three public diabetic clinics were randomly assigned to three study groups and then 45 participants were randomly recruited from every clinic. 24-week educational interventions via WhatsApp included a self-regulation-based intervention (SRIP), a combined mindfulness and self-regulation intervention (CMSRIP), and routine care with training on COVID-19 prevention were performed in two intervention groups and a control group respectively. Data were collected at baseline and four and 16 weeks using a Mindfulness-based questionnaire (MSCS) and a summary of the diabetes self-care activities scale (SDSCA). The SPSS software (V. 22) was used to analyze data through ANOVA test, chi-square tests, one-way ANOVA, Levene's test, and Repeated measures MANCOVA at a significance level of less than 0.05. The total scores of MSCS and SDSCA in four and 16 weeks significantly increased compared to the baseline in both SRIP and CMSRIP groups (P < 0.001), but not in the control group. The mean scores of MSCS and SDSCA at week 4 in CMSRIP group improved significantly more than SRIP group (98.43 ± 5.83 vs. 90.31 ± 12.62 and 70.38 ± 4.18 vs. 62.35 ± 5.53). Combined mindfulness and self-regulation education are significantly effective in promoting self-care skills and improving cognitive processes and can used to improve older adults' self-care behaviors.