Pericardial cysts are rare benign tumors often discovered incidentally during radiological examinations. Despite being predominantly asymptomatic, they can cause significant clinical manifestations and complications. The article describes a case of migrating pericardial cyst that was initially misinterpreted as left-sided encysted pleural effusion. Detailed assessment of dynamic changes on multislice computed tomography and subsequent surgical treatment revealed that the neoplasm originated from the pericardial layers altering its location, shape, and size. Awareness of the phenomenon of pericardial cyst migration and the use of up-to-date imaging techniques play a crucial role in accurate diagnosis and selecting optimal treatment strategies.