We undertook a breeding program to produce a white button mushroom cultivar with high temperature tolerance in preparation for climate change. The results were as follows. The strains KMCC00540, KMCC00591, and KMCC00643 were selected, and homokaryons were selected and hybrids were identified with ISSR and SSR markers, respectively. The selected hybrids were cultivated in three repetitions at a temperature of 20-25 °C and a humidity of 80% or higher. The variation in agricultural traits and fruiting body characteristics was least in the Abs4-2016-121 strain. This line is a hybrid of KMCC00591 and KMCC00875, and field experiments at Gyeongju and Buyeo farms found that the Abs4-2016-121 line has high fruiting body with hardness and delayed opening of pileus at high temperatures, so it was bred as a high-temperature cultivar named ‘Hadam’.