The chapter raises the question as to whether the socio-religious current we now call “Tantrism” may represent a wider Monsoon Asian areal phenomenon, with roots in a past that predates the common era. Being directly inspired by, and engaging with, the work of Robert Dentan, this chapter evaluates his theory of a common religious base in South and Southeast Asia. It focuses on the hypostases of the Vedic god Rudra-Śiva and the related Śaiva and Śākta Tantric traditions that arose in South Asia in the mediaeval (Post-Gupta) period, and adds evidence from premodern and modern South Asian, Javano-Balinese, and Malay contexts. The presence of similar religious tropes and a shared vocabulary suggest long-standing historical connections between South and Southeast Asian societies, which cannot always be explained in terms of influences of the former over the latter but are rather suggestive of a complex process of exchange, and perhaps of a common ancestry.