Background <p>Calpain is one of the intracellular calcium-activated cysteine proteases and exerts pivotal functions in many diseases. The aim was to explore the relationship of serum calpain-5 concentration and Coronavirus disease 2019 (COVID-19) via a cohort study.</p> Methods <p>The whole 338 COVID-19 patients were selected. Clinical features and demographic information were collected. The concentration of serum calpain-5 was measured via enzyme-linked immunosorbent assay (ELISA).</p> Results <p>Serum calpain-5 concentration was upregulated with increased severity scores, including SMART-COP, 4&#xa0;C-Mortality, CURXO, A-DROP, DTPNCP, COVID-GRAM, PSI, and CURB-65 in COVID-19 patients. Spearman correlation analyses suggested there were tight correlations between serum calpain-5 level and numbers of clinical features. Additionally, multivariate linear and logistic regression models showed that serum calpain-5 level was positively associated with severity scores. Moreover, the frequencies of mechanical ventilation, vasoactive agent usage, ICU admission, and death during hospitalization within 30 days were grown with elevated serum calpain-5 concentration on admission. Logistic regression analyses demonstrated that serum higher calpain-5 on admission elevated the risk of adverse prognosis during hospitalization within 30 days.</p> Conclusions <p>The concentration of serum calpain-5 is positively correlated with disease severity and adverse prognosis, implying calpain-5 may be involved in the progression of COVID-19. Serum calpain-5 level may served as a useful biomarker for guiding clinicians in managing and monitoring disease outcome in COVID-19 patients.</p> Clinical trial registration <p>Clinical trial number: Not applicable.</p>