Live-cell imaging of mRNA enables tracking of mRNA localization and its dynamics in real time. This is fundamentally important in understanding how cells use RNA to regulate gene expression and orchestrate biological processes. Here, we describe a method of using an engineered RNA tag, called Pepper RNA tag, to visualizing mRNA in living cells. In this method, an mRNA of interest engineered to contain the Pepper RNA tag turns on the fluorescence signals of fluorogenic proteins, which enables tracking of mRNA in living cells.