Although H. P. Blavatsky (1831-1891), co-founder of the Theosophical Society, has featured prominently in histories of Western esotericism, her engagement with late nineteenth-century comparative religion has not been appreciated. This thesis offers the first sustained analysis of H. P. Blavatsky's theosophical comparative religion. Despite the fact that one of the original goals of the Theosophical Society was advancing comparative religion, H. P. Blavatsky has been excluded from standard accounts of the field. This thesis draws on a range of theoretical resources - Richard Rorty's pragmatic theory of knowledge, Alun Munslow's analysis of narrative in history, Thomas Gieryn's critique of boundary-making in science, and Lorraine Daston and Peter Galison's history of objectivity - to argue for the inclusion of H. P. Blavatsky in the history of comparative religion. Substantial chapters analyse H. P. Blavatsky's major works, from Isis Unveiled (1877) to The Secret Doctrine (1888), to uncover the theoretical template that she developed for analysing religion and comparing religions. The thesis highlights H. P. Blavatsky's interpretative strategies in fashioning a theosophical comparative religion. In developing a comparative religion, H. P. Blavatsky referred to leading figures in the emerging field of the academic study of religion, such as F. Max Müller, E. B. Tylor, and Herbert Spencer, in positioning her theosophical comparative religion in the context of late nineteenth-century production of knowledge about religion and religions. This thesis demonstrates that H. P. Blavatsky's comparative religion was reasoned, literary, rhetorical, coherent, and strategic. By analysing H. P. Blavatsky's theoretical work on religion and religions in its late nineteenth-century context, this thesis contributes to the ongoing project of broadening our understanding of the complex and contested history of the study of religion.