The red-barred anthias Pseudanthias rubrizonatus is a common tropical deep reef fish species found in Australia, but little is known about its dietary preferences and trophic interactions. We examined the gut contents and stable isotope signatures (δ13C and δ15N) of P. rubrizonatus from populations on the North-West Shelf of Australia to determine differences in diet relative to site, depth and fish size. We sampled 5 fish populations from a series of sub-sea structures, from 82 to 152 m depth, which had been submerged for up to 15 yr. Gut content analysis suggested that P. rubrizonatus displays an opportunistic feeding strategy and utilises both pelagic and benthic resources, including larval fishes, heteropods, isopods and mysids. Stable isotope analyses revealed that at all depths P. rubrizonatus underwent an ontogenetic diet shift. Values for δ13C in muscle ranged from –19.7‰ for small fish to –16.2‰ for larger individuals, and δ15N ranged from 8.2‰ for smaller fish to 13.2‰ for larger fish, indicating that a diet shift occurs at the end of juvenile development between 30 and 50 mm standard length. By simultaneously analysing gut contents and stable isotope signatures of the collected specimens, we have documented opportunistic dietary strategies that may assist P. rubrizonatus to colonise isolated structures.