Thermosensitive polymers such as poly(N-isopropylacrylamide) (poly(NIPAM)) are soluble in water at low temperatures; however, they are insoluble above the intrinsic temperature of the polymer due to its transition to hydrophobicity. In this study, a novel dewatering method of activated sludge by utilizing the hydrophilic/hydrophobic transitions of thermosensitive polymers was investigated. The cationic thermosensitive copolymers of NIPAM and N,N-dimethylaminopropylacrylamide (DMAPAA) synthesized with various DMAPAA contents were used. The activated sludge and polymer solution were mixed well at room temperature. Then, the mixture was heated to the desired temperature. Subsequently, constant pressure filtration and expression processes were carried out. The filtration/expression apparatus was maintained at the desired temperature during these processes. When dewatering was performed at room temperature, the moisture content of the dewatered cake increased with the polymer dosage. This is because the sludge that was sufficiently covered with the adsorbed hydrophilic polymer molecules dispersed stably. The adsorbed polymer acts as a steric stabilizer. On the other hand, by carrying out dewatering above the transition temperature of the polymer, the moisture content of the dewatered cake decreased remarkably with an increase in the polymer dosage. The dewatering performance depended on the temperature and the DMAPAA content of the copolymer. Such high dewatering performance with thermosensitive polymers can be attributed to the hydrophobic interaction among the thermosensitive polymer molecules adsorbed on the sludge.