There are contradictory reports about the association of magnesium and attention- deficit hyperactivity disorder (ADHD). Moreover, some studies reported that magnesium is effective for treating ADHD. This is the first systematic review evaluating the effectiveness of magnesium to treat patients with ADHD. The two databases of MEDLINE/ PubMedline and Google Scholar were electronically searched using the search items and considering inclusion and exclusion criteria. Out of the 37 titles, only six studies were experimental interventional studies. However, three of these studies were without any control group. The other three studies were controlled clinical trials. Nevertheless, none of the studies was a randomized double- blind controlled clinical trial. These studies supported that magnesium is effective for treating ADHD. Magnesium monotherapy studies were not found. There is no well- controlled clinical trial investigating the efficacy and safety of magnesium for treating ADHD. In addition, the safety of magnesium in ADHD is ignored. Therefore, until further strong evidences for its efficacy and safety are provided, magnesium is not recommended for treating ADHD.