Coronavirus disease 2019 (COVID-19) can be fatal in severe cases. Accordingly, predicting the severity and prognosis of the disease is valuable. This study examined the role of electrolyte imbalances in predicting the severity of COVID-19. In this cross-sectional study, 169 hospitalized patients with COVID-19 were included and categorized into three groups based on the severity of the disease (moderate, severe, and critical). Serum levels of electrolytes (calcium [Ca], phosphorus [P], sodium [Na], potassium [k], and magnesium [Mg]), inflammatory markers (D-dimer, C-reactive protein [CRP], ferritin, and lactate dehydrogenase [LDH]), and 25OHVitamin D were measured. The mean age of patients was 53 years, and 54% were male. They had moderate, severe, and critical illnesses in 22%, 47%, and 31%, respectively. CRP, D-dimer, and ferritin increased with the severity of the disease. The lower median values of Mg, Na, 25OHVitamin D, Ca, LDH, and higher median lymphocyte counts were observed in the moderate vs. the severe group (P < 0.05). These parameters have acceptable sensitivity and specificity at the suggested cut-off level to discriminate the moderate and critical cases. Serum parameters introduced in this study are appropriate for differentiating between critical and moderate cases. The electrolyte imbalance can predict critical patients.