Heat shock transcription factor (HSF) is one of the most important regulatory elements in plant development and stress response. Rhohomyrtus tomentosa has many advantages in adapting to high temperature and high humidity climates, whereas its inherence has barely been elucidated. In this study, we aimed to characterize the HSF family and investigate the thermal adaptation mechanisms of R. tomentosa. We identified 25 HSF genes in the R. tomentosa genome. They could be classified into three classes: HSFA, HSFB, and HSFC. Gene duplication events are major motivations for the expansion of the RtHSF gene family. Most of the genes in the same subclass share similar conserved motifs and gene structures. The cis-acting elements of the promoter regions of RtHSF genes are related to development, phytohormone signaling, and stress responses, and they vary among the genes even in the same subclass, resulting in different expression patterns. Especially, there exists subfunctionalization in the RtHSFA2 subfamily in responding to various abiotic stresses, viz. RtHSFA2a is sensitive to drought, salt, and cold stresses, whilst RtHSFA2b is mainly induced by heat stress. We further proved that RtHSFA2b might be of more importance in R. tomentosa thermotolerance, for Arabidopsis plants with overexpressed RtHSFA2b outperformed those with RtHSFA2a under heat stress, and RtHSFA2b had much higher transcription activity than RtHSFA2a in regulating certain heat shock response (HSR) genes. RtHSFA2a plays a role in transactivating RtHSFA2b. All these results provide a general prospect of the RtHSF gene family and enclose a basal thermal adaptation mechanism of R. tomentosa.