Study Objectives: The purpose of this study is to conduct a systematic review and meta-analysis evaluating the effects of respiratory muscle therapy (ie, oropharyngeal exercises, speech therapy, breathing exercises, wind musical instruments) compared with control therapy or no treatment in improving apneahypopnea index ([AHI] primary outcome), sleepiness, and other polysomnographic outcomes for patients diagnosed with obstructive sleep apnea (OSA). Methods: Only randomized controlled trials with a placebo therapy or no treatment searched using PubMed, EMBASE, Cochrane, and Web of Science up to November 2018 were included, and assessment of risk of bias was completed using the Cochrane Handbook. Results: Nine studies with 394 adults and children diagnosed with mild to severe OSA were included, all assessed at high risk of bias. Eight of the 9 studies measured AHI and showed a weighted average overall AHI improvement of 39.5% versus baselines after respiratory muscle therapy. Based on our meta-analyses in adult studies, respiratory muscle therapy yielded an improvement in AHI of −7.6 events/h (95% confidence interval [CI] = −11.7 to −3.5; P ≤.001), apnea index of −4.2 events/h (95% CI = −7.7 to −0.8; P ≤ .016), Epworth Sleepiness Scale of −2.5 of 24 (95% CI= −5.1 to −0.1; P ≤ .066), Pittsburgh Sleep Quality Index of −1.3 of 21 (95% CI= −2.4 to −0.2; P ≤ .026), snoring frequency (P = .044) in intervention groups compared with controls. Conclusions: This systematic review highlights respiratory muscle therapy as an adjunct management for OSA but further studies are needed due to limitations including the nature and small number of studies, heterogeneity of the interventions, and high risk of bias with low quality of evidence.