This dissertation has three main research questions. First, how did the Tambralinga Kingdom develop? Second, what was the significance of this kingdom in maritime Southeast Asia? And, third, what was the kingdom's cultural geography? To answer these questions, the author reviewed the previous scholarly work on the topics and conducted a series of archaeological surveys and excavations, ethnographic interviews, and studies of historical records, such as stone inscriptions and old chronicles. In terms of its development, Tambralinga, located in Peninsular Siam between the South China Sea and the Bay of Bengal, had the openness of an island to trade and cultural influences. It originally emerged as a trade station in the Trans-Asiatic Trade Network in the early centuries CE and became a powerful kingdom with its peak in the 13th century. Its heartland was situated on the coastal lands of Nakhon Province, Southern Thailand. The late prehistoric fishing-trading communities on this coast were active in the exchange network in the South China Sea, suggested by the fact that this area had the highest density of Bronze Drums in the Malay Peninsula in the late centuries BCE. By at least the 5th century CE, Tambralinga seems to have developed into a state-level polity with Visnu images, lingas, and possible stone inscriptions, all of which point to the existence of Hindu shrines. These may have been rather small and made in part of perishable materials. Its heartland has the highest density of the earliest Visnu images of Southeast Asia, called "the conch on the hip group" dated to c. the 5th century CE and reflected the advanced socio-political development in the area. Tambralinga would seem to have been served as a center of innovation of these Visnu images in maritime Southeast Asia. The heartland also has the highest density of stone inscriptions and Hindu shrines in the isthmian tract in the period between the 5th and 11th century CE, the period called "the Early Tambralinga Period" in the dissertation. The locations of the shrines suggest the distribution of communities and communication routes along the coast of Nakhon. Five clusters of shrines are identified in this work. The center of the early Tambralinga Kingdom was likely situated in the area between the Tha Khwai, Tha Chieo, and Tha Thon Rivers. In the late 8th century, Tambralinga was associated with Srivijaya, an entity that could also be viewed as a league of trading polities, which focused on trade and commercial collaboration, rather than agonistic engagement. The peak of the Tambralinga Kingdom seems to be in the 13th century. A large number of Chinese ceramics dated to the 13th to 14th centuries were discovered by the author in the excavations in 2009 at Nakhon City, the capital of the Tambralinga Kingdom in this period. In terms of cultural geography, the heartland of Tambralinga had beach ridges running in the north-south direction as the core of its landscape. This strip of the coastal land offered easy travel and exchange. Compared to a house, Tambralinga had its gate opened up to the South China Sea and had the mountain in its backyard. The mountains were important to the kingdom's trade and development as it was the source of exotic goods, such as forest products and tin, highly valued by foreign merchants. Situated between the shores and the mountains was the flood plain that produced rice and cattle for the population in the kingdom. Rivers and walking trails provided passageways between ecological zones in the kingdom. They also connected the kingdom to the west coast of the isthmus.