Inhalation phage therapy is proposed as a replacement approach for antibiotics in the treatment of pulmonary bacterial infections. This study investigates phage therapy on bacterial pneumonia in patients with moderate to severe COVID-19 via the inhalation route. In this double-blind clinical trial, 60 patients with positive COVID-19 hospitalized in three central Mazandaran hospitals were chosen and randomly divided into two intervention and control groups. Standard country protocol drugs plus 10 mL of phage suspension every 12 h with a mesh nebulizer was prescribed for 7 days in the intervention group. The two groups were compared in terms of O2Sat, survival rate, severe secondary pulmonary bacterial infection and duration of hospitalization. Comparing the results between the intervention and control group, in terms of the trend of O2Sat change, negative sputum culture, no fever, no dyspnea, duration of hospitalization, duration of intubation and under ventilation, showed that the difference between these two groups was statistically different (P value < 0.05). In conclusion, inhalation phage therapy may have a potential effect on secondary infection and in the outcome of COVID-19 patients. However, more clinical trials with control confounding factors are needed to further support this concept.