Background: Obstructive sleep apnea syndrome (OSA) is a common but often under diagnosed condition. According to literature, OSA prevalence in atrial fibrillation (AF) patients varies from 21 to 85%. OSA is increasingly recognized as a risk factor for biventricular dysfunction. The present study aimed to compare left and right ventricular functions, assessed by conventional echocardiographic parameters and speckle tracking imaging, in non-valvular atrial fibrillation (NVAF) patients with and without severe OSA. Methods: A cross-sectional analytic study was conducted. Forty successive patients with NVAF were included. All of them had a clinical screening for symptoms suggestive of OSA and underwent polysomnographic study. Patients were divided into two groups (group 1: without severe OSA with an apnea-hypopnea index (AHI) < 30 events per hour (e/h), and group 2: having severe OSA with an AHI ≥ 30 e/h). Echocardiography was performed in all patients. Left and right ventricular function parameters were measured including global longitudinal strain (GLS) and myocardial performance index (MPI). Results: OSA was diagnosed in 90% of NVAF patients. The average AHI was 22.1 ± 13 e/h. Eleven patients (27.5%) had mild OSA, 9 patients (22.5%) had moderate OSA, and 16 patients (40%) had severe OSA. General clinical characteristics were comparable between groups. A statistically significant association was demonstrated between severe OSA and impairment of left ventricular GLS (-17.3 ± 4.5 vs. -14.9 ± 3%, in group 1 and 2 respectively, p = 0.02) and left ventricular MPI (0.37 ± 0.09 vs. 0.49 ± 0.13, in group 1 and 2 respectively, p = 0.01). Right ventricular lateral wall strain was non significantly lower in group 1 compared to group 2 (-22.5 ± 8.4 vs. -18.4 ± 5.8%, in group 1 and 2 respectively, p = 0.15). On multivariate logistic regression analysis, left ventricular GLS impairment (> -18%) and MPI > 0.37 were independent predictors of severe OSA. Conclusion: Severe OSA was diagnosed in 40% of NVAF patients. Impairment of left ventricular GLS and left MPI were statistically associated with severe OSA.