Background Obstructive sleep apnea (OSA) syndrome is a chronic disease characterized by episodes of upper airway collapse and has been associated with increased cardiovascular morbidity. Clinical and epidemiological studies have shown that OSA and obesity are strongly associated. Aim To assess the extent of reduction of the retroglossal (RG) transverse pharyngeal diameter during Muller maneuver (MM) in patients with OSA and its relation to the degree of OSA and their possible cutoff values for the diagnosis of OSA and its severity. Patients and methods A total of 288 adults were included on the basis of having at least one of the major OSA symptoms. Polysomnography was performed to confirm diagnosis and classify patients into mild, moderate, and severe groups regarding apnea–hypopnea index. Anthropometric indices such as BMI and neck circumference (NC) were measured for all included patients. Submental ultrasound (US) was performed to measure RG transverse pharyngeal diameter airspace during normal tidal breathing and during MM. Results BMI, apnea–hypopnea index, and NC were significantly higher in severe OSA than mild to moderate or non-OSA patients. Regarding US parameters, tidal RG diameter and post-MM RG diameter were shortest in patients with severe OSA, whereas % decrease in RG diameter after MM was largest in severe OSA than mild–moderate and non-OSA patients, with significant difference. Moreover, the decrease in RG diameter after MM is the most valuable in prediction of OSA and detecting severity (predicting OSA at cutoff value >27%, mild OSA at cutoff value of >27–30%, moderate OSA at cutoff value of >30–38%, and lastly severe OSA at cutoff value >38%). Conclusion The % of RG shortening during MM and NC is an independent predictor of OSA. Thus, concomitant submental US and NC may be used as a promising screening tool for OSA and its severity.