Ichthyophis bannanicus is a terrestrial caecilian, and little is known about many aspects of the ecology of this species. We investigated the correlation between diet and body size, spatiotemporal variation in dietary composition, diversity index of prey, and size dimorphism among populations for this species. Specimens (N = 135) were collected from May 2010 to April 2011 in the Mekong Delta, Vietnam. We found 178 prey items representing 11 unique families and discovered that the diet of I. bannanicus is composed mainly of small prey; the most important were Achatinidae, Lumbricidae, and Formicidae, with a combined importance index of 77%. Diet composition, prey size, and total prey volume in I. bannanicus changed between dry and rainy seasons and among regions (P < 0.001). Mean prey volume increased with body size and showed positive correlations (P < 0.001) between total length and mouth width and prey size consumed. The total dietary breadth of I. bannanicus was 4.52, and all six populations of the Bannan Caecilian were found to consume snails, earthworms, ants, and termites. Results indicated that precipitation, but not temperature, was associated positively with prey volume. All collected I. bannanicus were adult females; we did not find male individuals of this species in the study area.