New Caledonia has an exceptionally diverse and unique flora, and there is growing concern about the impacts of introduced wild rusa deer on native forests. The diets of free-ranging rusa deer from two native forest sites were studied using rumen content analysis. Samples (n = 61) from a sclerophyll forest site consisted principally of graminoids (64.6 ± 4.4% dry weight), mainly native grass, but the proportion of woody species increased during the dry season. In the rumen samples from the rainforest site (n = 56), woody species were the predominant plants (61.9 ± 3.7% dry weight), and the composition of the diet was more constant across seasons. Most of the food items found could not be identified to species level, but it is estimated that native plants represent 40–60% of rusa deer diet. Although more work is needed to assess diet preferences and impacts of rusa deer, we suggest that they represent a potentially important threat to some native plant species in New Caledonia.