Arachnoid cysts, although usually asymptomatic, can be associated with psychiatric symptoms, including delusions and hallucinations. The role of neuroimaging findings, particularly arachnoid cysts, and their influence on psychiatric symptoms is still controversial and debated. We present the case of a 56-year-old male who sought medical help for his psychotic symptoms, mainly paranoid delusions, and auditory and tactile hallucinations. Brain imaging studies at the time of admission revealed a large left frontoparietal arachnoid cyst. The patient was then started on psychiatric medication but did not show any sign of clinical improvement. After discussing the case with the patient´s neurosurgeon, it was decided to submit the patient to drainage of his arachnoid cyst. The patient was reevaluated two weeks after the procedure showing significant clinical improvement, particularly in his positive psychotic symptoms. The rapid improvement of the psychotic symptoms after removing the cyst suggests that, at least in part, the mass was contributing to the symptoms presented.