Hydatid cyst is known to affect all possible anatomical locations of the human body. However, the mediastinal localization is extremely rare. This benign, commonly asymptomatic and incidentally detected disease, at times may simulate an aggressive malignancy by its potential to cause osseous destruction and intraspinal extension. A young female, farmer by occupation, presented with complaints of left chest pain and monoparesis of the left lower limb. Radiograph followed by computed tomography (CT) of the chest demonstrated a cystic mass within the posterior mediastinum, eroding and scalloping overlying ribs and extending into the spinal canal by causing destruction of adjoining vertebra, and assuming a dumbbell shape. The serology was positive for echinococcosis. The patient underwent surgery and the postoperative histopathology confirmed the diagnosis of hydatid cyst. The patient recovered with no complications or recurrence. Hydatid cyst should always be considered in the differential diagnosis of mediastinal cystic lesions, however aggressive the lessions may appear.