Background: Pneumonia is one of the most common serious infections in children. Scoring systems have been adopted to quantify the severity of the disease, but they were based on clinical findings that can vary according to the subjective assessment of the clinician. We hypothesized that diaphragm ultrasound (DUS) parameters may be a new useful tool to objectively score the severity of the disease and predict outcomes in children with pneumonia. Methods: Children diagnosed with pneumonia, aged between 1 month and 18 years, were prospectively evaluated in the pediatric emergency department. The Pediatric Respiratory Severity Score was used to indicate the severity of the disease and DUS was performed. Diaphragm thickness at the end of inspiration and expiration, thickening fraction (TF), diaphragm excursion, inspiratory slope (IS), expiratory slope (ES), and total duration time of the respiratory cycle were calculated. Results: There were 96 patients enrolled in the study. Inspiratory slope and ES measurements had positive correlations with respiratory rate and length of stay in the hospital and negative correlations with oxygen saturation levels. Furthermore, TF values were negatively correlated with respiratory rate and length of stay in the emergency department. Patients with higher clinical scores had increased IS and ES and decreased TF values. Conclusion: Diaphragm ultrasound can be a promising and useful tool to assess diaphragmatic dysfunction in patients diagnosed with pneumonia. Diaphragm parameters, especially TF, IS, and ES, may provide objective and reliable information to predict the severity of the illness, the need for respiratory support, and outcomes.