Background:Several randomized clinical trials have evaluated the efficacy of prophylactic magnesium (Mg) supplementation in prevention of post-operative atrial fibrillation (POAF) in patients undergoing cardiac artery bypass grafting (CABG). We aim to determine the role of prophylactic Mg in 3 settings (intraoperative, postoperative, intraoperative + postoperative) in prevention of POAF. Methods:A systemic literature search was performed (until January 19, 2019) using PubMed, EMBASE, Web of Science, and Cochrane Central Register of Controlled Trials to identify trials evaluating Mg supplementation post CABG. Primary outcome of our study was reduction in the POAF burden post CABG. In order to assess possible differences in the timing of Mg administration, we further divided trials into three subdivisions (secondary outcomes): intraoperative, postoperative and a combination of intra-and postoperative administration of Mg.  Results: We included a total of 2,430 participants (1,196 in the Mg group and 1,234 in the placebo group) enrolled in 20 randomized controlled trials. Pooled analysis demonstrated no difference between Mg supplementation and placebo groups for reduction in POAF (RR 0.90; 95% CI, 0.79-1.03; p=0.13; I2=42.9%). A significant reduction in POAF was observed with postoperative magnesium supplementation (RR 0.76; 95% CI, 0.58-0.99; p=0.04; I2=17.6%) but not with intraoperative or intraoperative plus postoperative Mg supplementation (RR 0.77; 95% CI, 0.49-1.22; p = 0.27; I2=49% and RR 0.92; 95% CI, 0.68-1.24; p = 0.58; I2=51.8%, respectively). Conclusions:Prophylactic magnesium supplementation in the postoperative period demonstrated a reduction in POAF in patients undergoing CABG.