Stress responses are a key feature of normal physiology and are usurped by cancer cells to ensure enhanced protein synthesis for growth, to compensate for genomic instability, and to protect cancer cells from therapy induced stress. Heat shock factor-1 (HSF1) is a major stress-response transcription factor, and its activity is markedly enhanced in cancer. Stress induces HSF1 conformational changes and post-translational modifications, leading to assembly of active HSF1 trimers that bind DNA to control target gene expression. Although the function of HSF1 in transcription is relatively well-known, the mechanisms leading to HSF1 activation and enhanced stress response in tumours are unclear. To investigate whether HSF1 is a primary sensor of proteotoxic stress in vivo, we studied the range of conditions that can cause HSF1 activation in vitro and in cells, and conditions that prevent its activation. We show that purified recombinant HSF1 adopts a stable monomeric conformation in vitro. Heat stress caused a conformational change and the assembly of HSF1 trimers. Conditions leading to protein denaturation, including heat stress, crowding, Hsp90 inhibition, or proteasome inhibition, all directly lead to HSF1 activation. In contrast, HSF1 activation in vivo is prevented by proteosynthesis inhibition, which reduces the amount of denatured proteins in the cell. These results establish that HSF1 is a direct sensor of proteotoxic stress, independent of post-translational modification, where abrupt environmental changes that cause protein denaturation simultaneously induce a conformational change in monomeric HSF1 leading to its activation. This mechanism explains the universal ability of cells to respond to proteotoxic stress and trigger a protective response when increased chaperone activities are required to restore homeostasis.