A simple copolymer, poly(NIPAM-co-BODIPY), consisting of N-isopropylacrylarnide (NIPAM) and boradiazaindacene (BODIPY) units, behaves as a fluorescent thermometer in water. The copolymer exhibits weak fluorescence at < 23 degrees C, but the intensity increases with a rise in temperature up to 35 degrees C, enabling an accurate indication of the solution temperature at 23-35 degrees C. The heat-induced fluorescence enhancement is driven by all increase in the polymer microviscosity, associated with a phase transition of the polymer from the coil to globule state. The viscous domain formed inside the globule-state polymer suppresses the rotation of the meso-pyridinium group of the excited-state BODIPY units, resulting in heat-induced fluorescence enhancement. The polymer shows reversible fluorescence enhancement/quenching regardless of the heating/cooling process and displays high reusability with a simple recovery process.