Sex change occurs as a usual part of the life cycle for many teleost fish. Changing sex is known to enhance the lifetime reproductive success of these fish and the modifications involved (behavioral, gonadal, morphological) are well studied. However, the exact mechanism behind the transduction of the environmental signals into the molecular cascade that underlies this singular transformation remains largely unknown. Cortisol is the main glucocorticoid in fish and the hormone most directly associated with stress. Recent research suggests that this hormone may act as a key factor linking social environmental stimuli and the onset of sex reversal by initiating a shift in steroidogenesis from estrogens to androgens. In this study, we aim to elucidate the role of cortisol in mediating sex change of a protogynous (female-to-male) hermaphrodite, the endemic New Zealand spotty wrasse (Notolabrus celidotus). We implanted slow-release cortisol pellets into female spotty wrasses to promote sex reversal under inhibitory conditions. We monitored each female daily for behavioral and morphological signs of sex change. To track the interrenal hormonal changes across the process of sex reversal, we collected blood samples fortnightly. We also obtained brain, pituitary and gonadal tissue to create a histological time series and to conduct transcriptome-wide expression analysis. We anticipate that this study will enhance our understanding of the role of cortisol in the initial stages of sex change, improve our understanding of sex determination and differentiation across vertebrates, and may lead to new tools to control fish sex ratios in aquaculture.