Aim Effects of heat shock transcription factor 1 (HSF1) deficiency on heat stress‐associated increase in slow soleus muscle mass of mice were investigated. Methods Both HSF1‐null and wild‐type mice were randomly assigned to control and heat‐stressed groups. Mice in heat‐stressed group were exposed to heat stress (41 °C for 60 min) in an incubator without anaesthesia. Results Significant increase in wet and dry weights, and protein content of soleus muscle in wild‐type mice was observed seven days after the application of the heat stress. However, heat stress had no impact on soleus muscle mass in HSF1‐null mice. Neither type of mice exhibited much effect of heat stress on HSF mRNA expression (HSF1, HSF2 and HSF4). On the other hand, heat stress upregulated heat shock proteins (HSPs) at the mRNA (HSP72) and protein (HSP72 and HSP110) levels in wild‐type mice, but not in HSF1‐null mice. The population of Pax7‐positive nuclei relative to total myonuclei of soleus muscle in wild‐type mice was significantly increased by heat stress, but not in HSF1‐null mice. Furthermore, the absence of HSF1 gene suppressed heat stress‐associated phosphorylation of Akt and p70 S6 kinase (p‐p70S6K) in soleus muscle. Conclusion Heat stress‐associated increase in skeletal muscle mass may be induced by HSF1 and/or HSF1‐mediated stress response that activates muscle satellite cells and Akt/p70S6K signalling pathway.