It has proven efficacy in reducing asthma exacerbations, but the effect size of montelukast (a leukotriene receptor antagonist) for varied severity of asthma exacerbations is not systematically assessed. This study was designed to systematically explore the evidence for montelukast, as first-line or add-on therapy, in preventing and treating asthma exacerbations in adult patients with asthma. Randomized controlled trials were searched in PubMed, CENTRAL, Web of Science, Embase, and OVID up to March 2013, where montelukast prevented or treated asthma exacerbations in adults. Primary outcomes were the number of patients experiencing exacerbations in chronic asthma and hospitalizations in acute asthma. Odds ratio (OR) with 95% confidence intervals (CI), risk difference, and number needed to treat (NNT) were calculated and pooled. Adverse events were also assessed in chronic asthma. Twenty trials for chronic asthma and six for acute asthma were identified. In comparison with placebo, adults with chronic asthma receiving montelukast had significantly reduced number of exacerbations (OR = 0.60 and 95% CI, 0.49, 0.74; NNT = 17 and 95% CI, 12, 29). However, montelukast was inferior to inhaled corticosteroids (ICSs) (OR = 1.63; 95% CI, 1.29, 2.0) and ICS plus long-acting beta2-agonist (LABA; OR = 3.94; 95% CI, 1.64, 9.48) as the first-line therapies and LABA (OR = 1.22; 95% CI, 1.05, 1.42) as the add-on therapies in reducing asthma exacerbations. In acute asthma, montelukast could statistically improve peak expiratory flow percent predicted (p = 0.008) and reduce systemic corticosteroid intake (p = 0.005). Montelukast had low risk in hoarseness and insomnia. Our meta-analysis suggests that montelukast significantly reduces mild, moderate, and part of severe exacerbations in chronic mild to moderate asthma, but it has inferior efficacy to ICS or ICS plus LABA.