To provide a novel lithium styrene sulfonate which is capable of solving a problem of an increase of the production cost due to drying under atmospheric pressure or reduced pressure at a temperature of at least 40° C. and a problem of polymerization of lithium styrene sulfonate. The lithium styrene sulfonate is characterized in that when measured by using a Thermogravimetric-Differential Thermal Analyzer under measuring condition of heating at a temperature raising rate of 2° C./min in a nitrogen stream, the temperature at the top of the main endothermic peak in a range of from 80 to 170° C., is at least 120° C.