Our goal in this study is to determine the prevalence and associated factors of obstructive sleep apnea (OSA) in morbidly obese patients undergoing bariatric surgery. This descriptive study was conducted at King Chulalongkorn Memorial Hospital from 2007 to 2015. Data of morbidly obese patients who underwent bariatric surgery were included using ICD-10 code for principle diagnosis "morbid obesity" (E668) and ICD-9 code for "bariatric surgery" (4389, 4438, 4439). Baseline characteristics of 238 patients who met the inclusion criteria demonstrated 49.2% male, mean age of 33.9 ± 10.8 years, and mean BMI of 52.6 ± 11.6. Sleeve gastrectomy and Roux-en Y gastric bypass surgery were performed in 51.5 and 48.5%; respectively. High risk for OSA using STOP-Bang as a screening questionnaire (≥3 points) was 92.7%. The prevalence of OSA using respiratory disturbance index (RDI) ≥ 5 was demonstrated at 85.7%. Mild, moderate, and severe OSA was observed in 8.8, 15.3, and 75.9%, respectively. Snoring, STOP-Bang score ≥ 3, fatty liver, and BMI were significantly correlated with OSA compared to the group without OSA with the odds ratio of 17.04 (p = <0.0001, 95% CI = 6.67-43.49), 16 (p = 0.01, 95% CI = 1.95-131.11), 4.75 (p = 0.001, 95% CI = 1.82-12.37), and 1.04 (p = 0.045, 95% CI = 1.0009-1.09), respectively. Comparison between non-severe and severe OSA groups demonstrated dyslipidemia and BMI to be correlated with OSA severity (odds ratio = 3.06, 95% CI 1.36-6.89, p = 0.007 and odds ratio = 1.07, 95% CI 1.03-1.13, p = 0.001, respectively). Obstructive sleep apnea is frequently observed in morbidly obese patients undergoing bariatric surgery and the severity tends to be severe. Snoring, STOP-Bang score ≥ 3, fatty liver, and BMI were significantly correlated with OSA. Dyslipidemia and BMI were demonstrated to be associated factors for severity of OSA in this population.