Heat shock factor Hsf1 of the yeast Saccharomyces cerevisiae binds to the heat shock element (HSE) of a subset of genes and activates their transcription in response to various environmental stresses. Hsf1 protein contains discrete domains respectively involved in DNA-binding, trimerization, transcription activation, and transcription repression. Here we have identified a novel domain rich in basic amino acids at the extreme C-terminus of Hsf1. Deletion or point mutations of the C-terminal basic region caused an inefficient heat shock response of genes containing noncanonical HSEs such as CUP1 and HSP26. The basic region is also essential for oxidative stress-inducible transcription of CUP1 by Hsf1. By contrast, it was dispensable for heat induction through the canonical HSE. We suggest that the basic region is a modulator involved in regulation of the Hsf1-mediated activation depending on the architecture of its binding site.