Pulmonary atelectasis is a disease characterized by the collapse of the pulmonary alveoli, leading to partial or total loss of function in the affected lung, and is mostly described in older dogs. It occurs due to chronic inflammatory and obstructive processes such as pneumonia. In infants who do not feed directly from the mother, it is common to develop aspiration pneumonia due to inhalation of a milk replacer. Once aspirated, food generates an inflammatory process in the airway that alters the function of the pulmonary surfactant, increases mucus production, and causes consequent airway obstruction, which may progress to alveolar and bronchial collapse. The aim of the present study was to report a case of a 4-month-old canine with pulmonary atelectasis secondary to bronchopneumonia and the outcome of the clinical case. The patient presented with cough, exercise intolerance, tachypnea, and progressive weight loss. Thoracic radiographic examination was performed to identify displacement of the mediastinum and heart overlapping the collapsed lung, interstitial-alveolar pattern associated with atelectasis of the middle lobe of the right lung, and a diffuse interstitial bronchial pattern throughout the other lung fields. Pulmonary support treatment was administered with antibiotics, bronchodilators, mucolytics, and inhaled corticosteroids, with resolution of clinical and radiographic symptoms after 60 days of treatment. Therefore, adequate treatment of chronic bronchopneumonia is effective in resolving pulmonary atelectasis and its associated clinical complications.