A four-year-old dachshund presented with a two-week history of pyrexia, depression, and cough. Four months earlier, the owner observed the dog swallow a whole popsicle stick, but the animal showed no clinical signs at that time. Radiography, ultrasonography, and computed tomography confirmed an intrathoracic linear foreign body and pleural effusion in the right thorax. The pleural fluid was bloody and purulent, and contained inflammatory cells and Escherichia coli. The dog was diagnosed with pyothorax induced by a foreign body, and was treated successfully by surgical removal of the foreign body, partial lung lobectomy, thoracic lavage, and antibiotics. The foreign body was identified as a popsicle stick that the dog had eaten.