Several lines of evidence link the canonical oncogene BCL6 to stress response. Here we demonstrate that BCL6 evolved in vertebrates as a component of the HSF1-driven stress response, which has been co-opted by the immune system to support germinal center formation and may have been decisive in the convergent evolution of humoral immunity in jawless and jawed vertebrates. We find that the highly conserved BTB corepressor binding site of BCL6 mediates stress adaptation across vertebrates. We demonstrate that pan-cancer cells hijack this stress tolerance mechanism to aberrantly express BCL6. Targeting the BCL6 BTB domain in cancer cells induces apoptosis and increases susceptibility to repeated doses of cytotoxic therapy. The chemosensitization effect upon BCL6 BTB inhibition is dependent on the de-repression of TOX, implicating modulation of DNA repair as a downstream mechanism. Collectively, these data suggest a form of adaptive non-oncogene addiction rooted in the natural selection of BCL6 during vertebrate evolution.