Thermoregulation is essential for homeothermic animals including mammals. In mammals, heat production, known as an adaptive mechanism for low temperature environments, is thought to mainly occur in specific tissues such as brown adipose tissue. However, there are yet many unclear aspects regarding the intracellular heat production and its mechanism. One of the main reasons is the lack of thermosensors that can measure the temperature inside the cell or in living animals. Recently, various types of fluorescent thermosensors, each based on distinct physical properties, that are able to visualize the intracellular temperature change in live cells have been developed. Among them, genetically-encoded (i.e. protein-based) fluorescent thermosensors especially possess high potentials as intracellular thermosensors, because these sensors can be easily targeted to the subcellular organelle directly where the heat production occurs. Importantly, using the protein-based thermosensors, existence of the heterogeneity of the intracellular temperature has been unveiled. In this review, we describe recent progress regarding the development of fluorescent thermosensors and the visualization of intracellular thermal dynamics in live cells.