The relationship between a population and its immediate natural landscape often shapes the course of social development and cultural practice. The increasing abundance of smelting byproduct in archaeological sites suggests that community-based metallurgical activities, established at least half a millennium earlier, intensified during the Iron Age (ca. 500 b.c.–a.d. 500) in central Thailand. Likely consequences of such a change would include higher demand for firewood, resulting in small-scale forest clearing, with subsequent altered access to wild terrestrial fauna. Tooth enamel from 13 human individuals dated to the Iron Age period of Promtin Tai were sampled for stable carbon isotope analysis to explore how people responded to anthropogenic landscape modification as reflected in the dietary regime. The data revealed that a C3-based dietary pattern was maintained throughout the Promtin Tai occupation. A slight enrichment of δ13C values from the Earlier to Later Iron Age may be attributable to increasing consumption of freshwater fauna, domestic fauna, resources collected from or fed on more opened fields, or any combination of these. A wider diversity of food items brought in via trade networks could also have contributed to the flexibility of foodways. The wealth of natural resources in prehistoric central Thailand facilitated the maintenance of a broad-spectrum diet that was also utilized elsewhere in tropical and subtropical Southeast Asia. This study highlights the key role of small sites, often connected by rivers and trade networks, in contributing to the understanding of prehistoric human ecology and cultural complexity in Mainland Southeast Asia.