Abiotic variables are thought to play a key role in mediating aggressive interactions between invasive and native species. What is less clear is how multiple variables, such as temperature and salinity, interact to influence interactions, which is problematic given both the current and future variability of abiotic factors. Whilst invasive fish are often more aggressive at higher temperatures, this could be negated at high salinities, depending on osmoregulation. In aquatic systems, one of the most ubiquitous invasive freshwater fishes in Australia is the Eastern Mosquito fish (Gambusia holbrooki) which is responsible for a decline in native biodiversity. Despite some evidence that high salinity alone limits G. holbrooki‘s behavioural dominance, it is unclear how another key water parameter- temperature- interacts with salinity to influence this species’ interactions with native fish. We conducted a highly controlled laboratory experiment in which we examined the relative effects of temperature and salinity on the agonistic interactions of G. holbrooki and an important native freshwater species, the Australian Bass (Macquaria novemaculeata). We subjected individuals to two levels of salinity (15 ‰ and 35 ‰) and temperature (21 °C and 28 °C), in a multi-factorial design, and scored both intra and inter specific aggression. In G. holbrooki and M. novemaculeata, both intra and interspecific aggression increased with elevated temperature when salinity was low, but not when salinity was high. Although there was no correlation between body size and aggression in G. holbrooki, there was a positive correlation in M. novemaculeata, with larger individuals more aggressive to hetero-specifics when temperature and salinity were elevated individually, but not when they both high. These results illustrate the complex relationship between species specific thermal and salinity tolerance, body size and aggressive behaviour and suggest that G. holbrooki will continue to dominate warming inland water bodies where salinity does not increase.