Thin-film transistors (TFTs) are widely used in bioelectronics for application in detecting metabolites, nucleic acids, proteins, cellular activities, and electrophysiological signals. During the COVID-19 pandemic, TFT-based COVID-19 biosensors displayed excellent performances and high levels of potential for use in various applications. TFTs, such as oxide-based TFTs and graphene field-effect, organic field-effect, and organic electrochemical transistors, yielded promising results in detecting viruses, ribonucleic acid, and antigens and antibodies. This review summarizes biosensors based on TFTs for use in detecting SARS-CoV-2 and the related biomarkers and analyzes sensing strategies based on transistors with different channel materials. Finally, future directions for researches regarding the applications of TFTs in viral detection are proposed.