A transcription factor DREB2A functions as a key regulator not only in drought stress responses but also in heat stress (HS) responses, and activates expression of many abiotic stress-responsive-genes involved in drought and HS tolerance. HsfA3 is one of the most up-regulated heat-inducible genes in transgenic plants overexpressing DREB2A. In this study, the analyses of HsfA3 expression profile and the transactivation analysis of HsfA3 showed that the expression of HsfA3 was directly regulated by DREB2A under HS. Microarray analysis using transgenic plants overexpressing HsfA3 also showed that overexpression of HsfA3 induces many heat-inducible genes. Furthermore, we showed that thermotolerance of the HsfA3 overexpressors was increased, and that of the hsfA3 T-DNA tagged mutants was decreased. These results indicate that HsfA3 regulates expression of many heat-inducible genes in the transcriptional cascade downstream of the DREB2A stress-regulatory system and functions in acquisition of thermotolerance under the control of the DREB2A cascade.