Background: Asthma is a highly prevalent chronic respiratory disease associated with airway inflammation, variable expiratory airflow limitation, and respiratory muscle dysfunction. While pharmacological therapy remains the cornerstone of management, non-pharmacological interventions such as respiratory muscle training (RMT) have emerged as potential adjuncts to improve outcomes in asthmatic patients. Methods: This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline. PubMed, Scopus, and Web of Science databases were searched for randomized controlled trials (RCTs) and quasi-experimental studies published between 2000 and 2017. Studies were eligible if they included patients with asthma of any severity undergoing inspiratory muscle training (IMT) or expiratory muscle training (EMT) as stand-alone interventions. Data extracted included study design, participant demographics, intervention details, and outcomes such as inspiratory and expiratory muscle strength (PImax, PEmax), exercise capacity, dyspnea, lung function, health-related quality of life (HRQoL), and healthcare utilization. Results: Six studies met inclusion criteria, comprising five RCTs and one quasi-experimental study. IMT consistently improved inspiratory muscle strength (PImax) and reduced exertional dyspnea. Several studies demonstrated reductions in β2-agonist consumption and emergency department visits, whereas improvements in pulmonary function, expiratory muscle strength, and HRQoL were inconsistent. Conclusion: RMT, particularly IMT, appears to be a safe and beneficial adjunct to conventional asthma management. It enhances inspiratory muscle strength and reduces dyspnea, with potential to improve clinical outcomes. High-quality, large-scale trials are warranted to confirm these benefits and define optimal training protocols.