Estimating diet is often an important step in understanding and managing the impacts of ungulates, particularly for non-native species, but there is uncertainty about whether rumen contents should be assessed using macroscopic or microhistological methods or both. Introduced sambar deer (Cervus unicolor) have a large and increasing distribution in south-east Australia, and there is concern about their impacts on native and non-indigenous plant species. We estimated the diets of 102 sambar deer harvested during 2007–2009 using macroscopic and microhistological rumen analysis techniques. We identified 105 plant species in the diets of sambar deer, 61 identified with both techniques and 22 identified only macroscopically or only microhistologically. Rumen species richness was 70% greater using the microhistological technique. Estimates of taxonomic (i.e., monocot and dicot) and functional (i.e., fern, shrub—tree, forb, climber, grass, and grass-like) group composition by the 2 techniques were similar. Shrubs—trees dominated the diet (macroscopic, 49.7%; microhistological, 52.7%), followed by grasses (macroscopic, 22.7%; microhistological, 17.5%) and ferns (macroscopic, 20.6%; microhistological, 22.2%). We identified 9 non-indigenous plant species, 2 of which we identified using only the microhistological technique. We detected seeds of the weed blackberry (Rubus fruticosus aggregate), sometimes in large amounts, only with the macroscopic technique, whereas we detected foliage of that species with both techniques. Both techniques classified sambar deer as an intermediate mixed feeder closer to a concentrate selector—browser than a bulk and roughage feeder. However, both techniques detected seasonal differences in the percentages of taxonomic and functional groups in the diet; sambar deer were more grazers in autumn and more browsers in spring. Our results indicate that both macroscopic and microhistological techniques may need to be used when it is important to identify plant species in the diet, as is often the case for non-native ungulates. However, either technique can be used to estimate broader taxonomic and functional diet composition, including feeding type.