Histopathological examination is a widely acknowledged technique to assess the reproductive health of aquatic organisms, but it has never been applied to the tunicate Styela plicata, a potential indicator species of water quality. In this study, we examined the oocyte differentiation of S. plicata obtained from the eastern coast of the Gulf of Thailand in order to provide basic information for future assessment of its reproductive health. The mature gonad of S. plicata comprised several ovo-testicular convoluted tubes, in which each tube was divided into apical and terminal portions. The ovarian tissue is located in the apical part of the tunicate body and contained oocytes of various differentiation stages (asynchronous development type) consisting of the four phases namely oogonial proliferation phase, primary growth phase, secondary growth phase (secondary growth and full-growth stages), and post-ovulatory phase. Changes in the morphology of oocytes and follicular cells were described for each differentiation stage. In addition, we unexpectedly observed a high prevalence of atretic follicles (24.5%), which might indicate the oocyte damage by environmental stresses. These findings would be useful for future assessment of reproductive histology of S. plicata and other tunicate species from environmental perspectives.