MicroRNAs (miRNAs), small non-coding RNAs that regulate gene expression, have received increasing attention as potential biomarkers of different diseases, including viral infections. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and the disease it is causing, coronavirus disease (COVID-19), has affected health, society and life worldwide since its pandemic spread. Differential expression of miRNAs in COVID-19 patients compared to healthy controls and also between different severity grades of COVID-19 has been described in several recent studies. In this review, we discuss in detail studies that investigated miRNA expression in body fluids of COVID-19 patients. Several studies found a different miRNA expression profile in COVID-19 patients compared to controls but also in different severity grades of the disease. We compared the main findings of the studies in order to identify miRNAs that have been identified as differentially expressed by more than one study and could serve as diagnostic or prognostic biomarkers of COVID-19. Finally, we highlight the challenges and perspectives associated to the use of miRNAs as biomarkers of COVID-19.