This thesis casts a new light on the role of Southeast Asia in the ancient Indian Ocean World. It brings together data and approaches from archaeology and historical linguistics to examine cultural and language contact between Southeast Asia and South Asia, East Africa and the Middle East. The interdisciplinary approach employed in this study reveals that insular Southeast Asian seafarers, traders and settlers had impacted on these parts of the world in pre-modern times through the transmission of numerous biological and cultural items. It is further demonstrated that the words used for these commodities often contain clues about the precise ethno-linguistic communities involved in their transoceanic dispersal. The Methodology chapter introduces some common linguistic strategies to examine language contact and lexical borrowing, to determine the directionality of loanwords and to circumvent the main caveats of such an approach. The study then proceeds to delve deeper into the socio-cultural background of interethnic contact in the Bay of Bengal and the Indian Ocean as a whole, focusing on the oft-neglected Southeast Asian contributions to the cultural landscape of this region and addressing the nature of pre-modern contact between Southeast Asia and the different parts of the Indian Ocean Word. Following from that, the last three chapters look in-depth at the dispersal of respectively Southeast Asian plants, spices and maritime technology into the wider Indian Ocean World. Although concepts and their names do not always neatly travel together across ethno-linguistic boundaries, these chapters demonstrate how a closer examination of lexical data offers supportive evidence and new perspectives on events of cultural contact not otherwise documented. Cumulatively, this study underlines that the analysis of lexical data is a strong tool to examine interethnic contact, particularly in pre-literate societies. Throughout the Indian Ocean World, Southeast Asian products and concepts were mainly dispersed by Malay-speaking communities, although others played a role as well.