Rhododendron hainanense Merr. is a tropical flowering shrub with high ornamental and medicinal value, but high temperatures are a limiting factor for its landscape application. The bHLH transcription factor family regulates plant growth and development and is crucial for responses to various stresses; however, its mechanism for regulating high-temperature stress remains unclear. This study conducted a whole-genome analysis of R. hainanense, identifying 117 RhbHLH members with complete domains, unevenly distributed across 12 chromosomes. Using Arabidopsis thaliana (L.) Heynh. as a reference, the 117 RhbHLH genes were systematically categorized into 21 subfamilies. Conserved sequence analysis showed that motifs 1 and 2 form the characteristic bHLH domain, while motifs 3–6 exhibit significant conservation. Analysis of gene duplication events indicated that R. hainanense has eight pairs of segmental duplications, suggesting their role in potential adaptive evolution. Transcriptomic studies revealed that genes such as RhbHLH107 and RhbHLH49 were highly expressed in response to high temperatures, suggesting their involvement in regulating heat stress. Furthermore, the RhbHLH gene showed a strong correlation with common metabolites like flavonoids and jasmonic acid under high temperatures, indicating that these genes may participate in regulating jasmonic acid and flavonoids to mediate plant stress responses. This study lays the foundation for further research on the role of the bHLH gene family in plant heat tolerance and provides a basis for future applications in breeding and horticulture.