Arachnoid cysts are benign congenital malformations of the arachnoid which account for approximately 1.4% of the intracranial lesions. Although it is usually asymptomatic, it may be accompanied by headache, hydrocephalus and seizure. Psychiatric disorders associated with arachnoid cysts are rare. In this article, we present a giant arachnoid cyst with hypomania symptoms and marked cognitive impairment. A 44-year-old female patient was admitted to our outpatient clinic with a 4-year history of headache, nervousness and attention problems. Magnetic resonance imaging revealed a giant arachnoid cyst with a size of 5.5x10.5x12.5 cm was found in the left hemisphere of the patient. Considering the patient's irritability, increase in the amount of speech, flight of ideas, sleep disturbance and attention disorders, the diagnosis of hypomania was made. The neuropsychological tests showed that the speed of information processing, mental flexibility and attention functions decreased, and executive functions were impaired. The patient was consulted to the neurosurgery department. But no surgical treatment was offered. Drug therapy for hypomanic symptoms and cognitive impairment was planned, but could not be started since the patient did not attend to the follow-up exams. Albeit the lack of follow-up constitutes a limitation for our report, we believe that the size of the cyst, significant impairment of cognitive functions and the presence of hypomania symptoms might contribute significantly to the literature. Other cases with arachnoid cyst displaying cognitive impairment were summarized in our article.