OBJECTIVES: Serum levels of potassium (K(+)) appear to be significantly lower in severe cases of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and the clinical significance of this is unknown. The objective was to investigate whether hypokalemia acts as a biomarker of severity in coronavirus disease 2019 (COVID-19) pneumonia and is associated with major clinical outcomes. METHODS: A retrospective cohort study of inpatients with COVID-19 pneumonia (March 3 to May 2, 2020) was performed. Patients were categorized according to nadir levels of K(+) in the first 72h of admission: hypokalemia (K(+) </=3.5mmol/l) and normokalemia (K(+) >3.5mmol/l). The main outcomes were all-cause mortality and the need for invasive mechanical ventilation (IMV); these were analyzed by multiple logistic regression (odds ratio (OR), 95% confidence interval (CI)). RESULTS: Three hundred and six patients were enrolled. Ninety-four patients (30.7%) had hypokalemia and these patients showed significantly higher comorbidity (Charlson comorbidity index >/=3, 30.0% vs 16.3%; p= 0.02) and CURB65 scores (median (interquartile range): 1.5 (0.0-3.0) vs 1.0 (0.0-2.0); p= 0.04), as well as higher levels of some inflammatory parameters at baseline. After adjustment for confounders, hypokalemia was independently associated with requiring IMV during the admission (OR 8.98, 95% CI 2.54-31.74). Mortality was 15.0% (n=46) and was not influenced by low K(+). Hypokalemia was associated with longer hospital and ICU stays. CONCLUSIONS: Hypokalemia is prevalent in patients with COVID-19 pneumonia. Hypokalemia is an independent predictor of IMV requirement and seems to be a sensitive biomarker of severe progression of COVID-19.