All records of crustaceans, molluscs, polychaetes, echinoderms and fishes from Sydney Harbour were extracted from the Australian Museum database, and plotted onto a map of Sydney Harbour that was divided into four regions. Records were analysed according to the number of species, genera and families present and over 3000 species were recorded, approximately double to triple the number of species found in the neighbouring Hawkesbury River, Botany Bay and Port Hacking. We examined the rate of accumulation of records and species over time since the 1860s, which followed a stepwise pattern usually correlated with the research activity of specific curators at the Australian Museum. The high species richness of Sydney Harbour is probably the result of multiple factors including significant tidal flushing and the high diversity of habitats present. Not all parts of the harbour have been well sampled, however, and we highlight areas and habitats that should be prioritised for further biodiversity surveys. An Appendix is provided listing all species in the five groups of animals considered here. The present study highlights the remarkable richness of the Sydney Harbour fauna and provides a consolidated background to guide future management and research, and emphasises the values of museum collections.