Background <p>The basic helix–loop–helix (bHLH) transcription factors constitute one of the largest families in plants and play vital roles in growth, development, and environmental stress responses. However, their involvement in flower development and heat stress response in <i>Rhododendron simsii</i> remains unclear.</p> Results <p>A total of 116 bHLH genes were identified in the <i>R. simsii</i> genome and classified into 17 groups based on phylogenetic relationships with <i>Arabidopsis thaliana</i> bHLHs. Gene duplication analysis revealed that 57 genes originated from whole-genome duplication events, and 49 from ancient segmental duplications. Promoter analysis identified abundant cis-regulatory elements associated with light, gibberellin, abscisic acid, and stress responses, suggesting regulatory roles in flower development and stress adaptation. Expression profiling showed that 23 and 13 genes were specifically expressed in vegetative organs and flowers, respectively. Analysis of five flower developmental stages grouped 49 genes into four distinct expression patterns. Under high-temperature treatment, 12 flower-expressed genes exhibited altered expression, with nine being significantly upregulated. Functional annotation indicated that two genes, <i>RsbHLH53</i> and <i>RsbHLH59</i>, are orthologous to <i>Arabidopsis MYC2</i> and <i>JAM2</i>, which are key regulators of heat stress response.</p> Conclusions <p>This study provides the first comprehensive characterization of the bHLH gene family in <i>R. simsii</i> and identifies candidate genes potentially involved in flower development and thermotolerance. These findings lay a theoretical foundation for understanding the molecular mechanisms underlying heat tolerance and for future molecular breeding of heat-resistant <i>R. simsii</i> varieties.</p>