Objective <p>The aim of this study is to investigate the prevalence of Fibromyalgia Syndrome (FMS) in patients diagnosed with Obstructive Sleep Apnea Syndrome (OSAS) and to examine the effects of continuous positive airway pressure (CPAP) treatment on pain levels, symptom severity, functional level, depression, and the relationship between these parameters and polysomnographic data.</p> Methods <p>A total of 251 patients aged between 30 and 65 years, diagnosed with OSAS and exhibiting symptoms for at least one year before diagnosis, were evaluated. 26 patients were excluded. Polysomnographic and demographic data were recorded. Patients were assessed for FMS by using the American College of Rheumatology criteria published in the years 1990 and 2016. Those diagnosed with FMS were evaluated by using the visual analog scale (VAS), Fibromyalgia Impact Questionnaire (FIQ), Fibromyalgia Fatigue Scale (FFS), Epworth Sleepiness Scale (ESS), and Beck Depression Inventory (BDI) before and 3 months after CPAP treatment. Pain threshold measurements were performed by using an algometer.</p> Results <p>FMS was detected in 43 of the 225 patients diagnosed with OSAS (19.1%). Fifteen of the 43 patients with FMS were excluded from the study due to irregular CPAP usage, and 28 patients completed the 3-months CPAP treatment and exercise program. Significant decreases in VAS, FIQ, FFS, BDI, and ESS values, and significant increases in pain threshold values were observed in these patients after CPAP treatment and exercise (<i>p</i> &lt; 0.05).</p> Conclusion <p>CPAP treatment was associated with improvement FMS symptoms in patients with comorbidity of FMS and OSAS. This study revealed the importance of oxygenation and quality sleep in FMS.</p> Clinical trial registration number <p>NCT06595771-Date of registration:04.09.2024 retrospectively registered.</p>