We report a thirty-six-year-old woman with intellectual disability who was referred for evaluation of suspected obstructive sleep apnea. The initial clinical impression suggested a syndromic case, so comprehensive genetic testing was undertaken. Overnight polysomnography revealed a severe rapid eye movement–predominant obstructive sleep apnea syndrome with an apnea–hypopnea index of 31.9 events per hour, rapid eye movement apnea–hypopnea index of 113.8 events per hour, and lowest oxygen saturation of 66%. Treatment with continuous positive airway pressure improved respiratory and sleep quality indices and was well tolerated. Whole-exome sequencing identified a de novo splice site variant in the pleckstrin homology domain interacting protein gene (c.41-1G > A), confirming a molecular diagnosis of Chung–Jansen Syndrome. Chung–Jansen syndrome is a rare neurodevelopmental disorder caused by heterozygous pathogenic variants in the pleckstrin homology domain interacting protein gene, marked by developmental delay, intellectual disability, behavioral abnormalities, dysmorphism, and progressive obesity. PHIP influences central and peripheral pathways controlling satiety, pancreatic function, and body weight. Despite frequent reports of sleep problems, systematic evaluation of sleep-disordered breathing has been limited. This adult case provides the first polysomnographic confirmation in the syndrome, supporting proactive screening for obstructive sleep apnea—especially in those with obesity. Integrating genetic assessment into sleep care can reduce diagnostic delays and better guide therapy and prognosis.