Background: Diabetes Mellitus (DM) is a chronic disease that requires continuous management, including adherence to routine follow-up visits to monitor health conditions and prevent complications. Adherence to routine follow-up may be associated with patients’ knowledge and family support. Purpose: To analyze the relationship between family support and knowledge of Diabetes Mellitus and adherence to routine follow-up visits among patients with DM at Batang 2 Public Health Center. Method: This quantitative study used a cross-sectional design. The sample consisted of 51 patients with DM selected using purposive sampling. Data were collected using questionnaires assessing family support, knowledge of DM, and adherence to routine follow-up visits. Data were analyzed using the Spearman rank correlation test with a significance level of α = 0.05. Results: The results showed a significant positive correlation between family support and adherence to routine follow-up visits (r = 0.649; p < 0.001), indicating a strong correlation. A significant positive correlation was also found between knowledge of DM and adherence to routine follow-up visits (r = 0.499; p < 0.001), indicating a moderate correlation. Conclusion: Family support and knowledge of Diabetes Mellitus were significantly and positively correlated with adherence to routine follow-up visits among patients with DM at Batang 2 Public Health Center. Strengthening diabetes education and involving family members in diabetes management should be considered as part of efforts to support patients’ adherence to routine follow-up visits.