We describe the diversity, composition, functional group organisation and biogeography of ants in Purnululu National Park (128°19 E, 17°19 S) in Western Australia's Kimberley region, located at a semi-arid/tropical zone boundary. Pitfall trapping collected a representative sample of ants from three main habitats within the park: spinifex, sandplain and gorges. In total, 154 species from 25 genera were collected. The richest genera were Melophorus, Iridomyrmex and Monomorium. Iridomyrmex was the most abundant genus, accounting for 80% of all individuals recorded in traps. Functional group composition was dominated by Dominant Dolichoderinae (41% of individuals collected) and Hot Climate Specialists (26%). Biogeographical composition was typical of semi-arid zone assemblages located on the overlap between Eyrean and Torresian zones: 50% of the species recorded represented Eyrean taxa, followed by 33% with Torresian affinities, and 10% Widespread. Differences in ant species richness, abundance and assemblage composition between the three habitat types were attributed to a combination of variation in vegetation structure and microhabitat composition. This paper provides baseline data on local ant communities in the semi-arid Kimberley, enabling a greater understanding of the conservation value of reserves within the Kimberley, one of Australia's least studied bioregions.