BACKGROUND AND AIMS: COPD is a common chronic condition in older age that impacts on daily activities and quality of life. Previous studies suggest that magnesium deficit in COPD patients affects bronco-obstruction, inflammation, and physical performance. We investigated whether oral magnesium supplementation in stable-phase COPD patients improves lung function, physical performance, and quality of life. METHODS: We conducted a double-blind randomized-controlled clinical study with 49 participants divided into two groups: one given 300 mg/day of magnesium citrate (n = 25) and the other one sachet/day of a placebo (n = 24). The following parameters were assessed at baseline and after 3 and 6 months: lung function (spirometry), physical performance (handgrip strength, lower limb strength, six-minute walk test), inflammation (e.g., C-reactive protein, CRP), disease-related symptoms, and quality of life (St George's Respiratory Questionnaire, EuroQoL-5D, the Modified British Medical Research Council Questionnaire). RESULTS: Linear mixed models revealed significantly lower CRP values in the intervention group than in the placebo group at the 6 month follow-up (β = - 3.2, 95% CI - 6.0, - 0.4, p = 0.03). Moreover, the maximum work for flexion tended to increase in both groups between the 3 and the 6 month assessments, especially in the placebo group. No significant differences within and between groups over the study period were observed for the other parameters tested. CONCLUSIONS: Although the established minimum sample size was not reached, our results suggests that oral magnesium supplementation may have a potential anti-inflammatory role. On the other hand, it does not seem to substantially influence lung function, physical performance, and quality of life in COPD patients. TRIAL REGISTRATION: The study is registered in clinicaltrial.gov (Trial Registration: NCT02680769, 13 June 2016, retrospectively registered).