Objective: Magnesium (Mg2+) deficiency can be observed in patients with chronic kidney disease (CKD) and is associated with increased inflammation, disease progression, and mortality. Dark chocolate is an excellent source of Mg2+. This study aimed to evaluate the effect of 70% cocoa chocolate intake on Mg2+ serum levels in patients with CKD on hemodialysis (HD). Methods: These are secondary analyses from a previous controlled pilot study. The study included 59 patients undergoing HD. Patients were allocated into 2 groups: dark chocolate and the control group. The dark chocolate group received 40 g of 70% cocoa chocolate for 2 months during HD sessions (3 times a week). The control group did not receive any intervention. Mg2+ was evaluated by a colorimetric test using a commercial kit (Bioclin®). NCT04600258 on March 1, 2020, retrospectively registered. Results: Thirty-five patients in the dark chocolate group (17 women, 53.4 ± 12.9 years) and 11 in the control group (4 women, 46.7 ± 10.9 years) completed the study. The median (interquartile range) overall Mg2+ serum levels were 2.4 (0.4) mg/dL, with no significant differences between the groups. After 2 months of supplementation, serum levels of Mg2+ increased significantly in the dark chocolate group from 2.5 (2.4-2.6) to 2.7 (2.6-2.8) mg/dL (P < .01). No change was observed in potassium and phosphorus plasma levels. Conclusions: Dark chocolate might be a promising nutritional strategy to improve Mg2+ levels in patients with CKD on HD. The offered dose was safe, not altering plasma phosphorus and potassium levels.