ABSTRACT: Background: Coronavirus disease (COVID-19) is currently the main public health problem worldwide. The administration of neutral electrolyzed saline, a solution that contains reactive species of chlorine and oxygen (ROS), may be an effective therapeutic alternative due to its immunomodulating characteristics, in systemic inflammation control, as well as in immune response improvement, promoting control of the viral infection. The present study evaluated the efficacy of treatment with intravenous and/or nebulized neutral electrolyzed saline combined with usual medical care versus usual medical care alone, in ambulatory patients with COVID-19. Methods: A prospective, 2-arm, parallel group, randomized, open-label, phase I-II clinical trial included 39 patients in the control group (usual medical care alone) and 45 patients in the experimental group (usual medical care + intravenous and/or nebulized electrolyzed saline, with dose escalation). Two aspects were evaluated during the twenty-day follow-up: i) the number of patients with disease progression (hospitalization or death); and ii) the Patient Acceptable Symptom State (PASS), a single-question outcome that determines patient well-being thresholds for pain and function. Biochemical and hematologic parameters, as well as adverse effects, were evaluated in the experimental group. Results: The experimental treatment decreased the risk for hospitalization by 92% (adjusted RR=0.08, 95% CI: 0.01-0.50, P=0.007), with a 43-fold increase in the probability of achieving an acceptable symptom state on day 5 (adjusted RR= 42.96, 95% CI: 9.22-200.0, P<0.001). Intravenous + nebulized administration was better than nebulized administration alone, but nebulized administration was better than usual medical care alone. Clinical improvement correlated with a decrease in C-reactive protein, and aberrant monocytes and an increase of lymphocytes, and platelets. Cortisol and testosterone levels were also evaluated, observing a decrease in cortisol levels and an increment of testosterone-cortisol ratio, on days 2 and 4. Conclusions: The experimental treatment produced no serious adverse effects. In conclusion, intravenous and/or nebulized neutral electrolyzed saline importantly reduced the symptomatology and risk of progression (hospitalization and death), in ambulatory patients with COVID-19. Trial registration: Cuban Public Registry of Clinical Trials (RPCEC) Database RPCEC00000309. Registered: 05. May 2020. https://rpcec.sld.cu/en/trials/RPCEC00000309-En.