Effects of heat stress on skeletal muscle mass in young and aged mice were investigated. Young (7-week) and aged (106-week) male C57BL/6J mice were randomly assigned to control and heat-stressed groups in each age. Mice in heat-stressed group were exposed to heat stress (41 °C for 60 min) in an incubator without anesthesia. Seven days after the exposure, soleus muscles were dissected from both hindlimbs. Protein content and the relative composition of Type II fibers in aged soleus were lower than those in young muscle. In aged soleus, higher baseline expression levels of HSP25, HSP72, and cathepsin L were observed compared with those in young muscle (p < 0.05). However, there were no significant differences in the expression levels of phosphorylated p70 S6 kinase (p-p70S6K), calpain 1, and calpain 2 of soleus between two age groups. A significant increase in muscle mass of both age groups was induced by heat stress (p < 0.05). Heat stress also upregulated the expressions of HSP25, HSP72, and p-p70S6K in both ages (p < 0.05). On the other hand, a significant decrease in cathepsin L expression by heating was observed in aged soleus, but not in young (p < 0.05). Both the percentage of Type I fibers and the expression of calpains in both age groups were unchanged following heat stress. Heat stress-associated downregulation of cathepsin L may be attributed to the upregulation of HSP72, which stabilizes lysosomal membranes (p < 0.05). Upregulations of HSP25, HSP72, and p-p70S6K and/or the downregulation of cathepsin L may play a role in heat stress-associated muscle hypertrophy in aged soleus muscle.