The successful reintroduction of eastern bettongs relies on suitable food being available, particularly truffles (underground fungal fruitbodies). Here, we compared stable isotopes (carbon (δ13C) and nitrogen (δ15N)) and radiocarbon (∆14C) in hair and scat in bettongs subsequently reintroduced to Mulligan Flat Woodland Sanctuary (MFWS) against marsupial species of different diets to test whether such measurements are useful for dietary assessments. Patterns in bettong scat δ13C reflected seasonal shifts in the consumption of photosynthate derived from C3 plants (dicots and temperate grasses) versus C4 plants (tropical grasses), whereas a winter peak in scat δ15N reflected maximum reliance on truffle consumption. In hair, eastern bettongs resembled preferential fungivores in δ13C and δ15N and differed from carnivores, grazers, herbivores and opportunistic fungivores. Radiocarbon levels were higher in fungivorous potoroos than in wallaby grazers, suggesting similar measurements in eastern bettongs could assess fungal importance in their diet. These isotopic measurements will be useful for assessing the diets of reintroduced bettongs and other fungivorous marsupials in new environments.