AimClimate change can have a broad range of impacts on the physiology and behaviour of animals. These effects can be mediated by the presence of other species in the community, but current forecasts of species responses to climate change largely ignore biological interactions. This is particularly true for novel interactions between range‐extending and native species, as this is often considered as noise and excluded from predictive models. Here we simulate how a tropical range‐extending and a local temperate fish species respond to the independent and combined effects of future ocean warming (RCPs 4.5 and 8.5) and novel ecological interactions in temperate ecosystems. LocationEast coast of Australia, along a ~ 2,000 km latitudinal gradient in a global climate warming hotspot. TaxonAbudefduf vaigiensis (tropical) and Atypicthys strigatus (temperate) fishes. MethodsWe use a dynamic energy budget model to simulate the length growth (i.e., increases in body length of individuals over time) and population persistence of juveniles of a tropical and a temperate fish species that form mixed‐species shoals, under different climate scenarios with and without the effects of novel ecological interactions. ResultsOur model forecasts that length growth of the juvenile tropical species will increase under ocean warming across subtropical to temperate regions. This increased length growth will be more drastic in temperate regions than in the subtropics, as winter warming will allow the tropical species to overwinter more frequently and show positive growth throughout the year. In contrast, warmer summer temperatures in the subtropics will likely exceed the optimal temperature of the juvenile temperate species at their trailing edge, resulting in reduced length growth under climate warming. Novel species interactions increased length growth of the juvenile tropical species but did not affect its winter or summer survival. In contrast, novel species interactions with tropical species were forecast to reduce length growth of the juvenile temperate species. Main ConclusionsOur study suggests that for some coastal fish species future warming will likely reverse body size dominance between temperate and tropical fish species, with increased novel interactions in temperate ecosystems (due to range extensions) but decreased novel interactions in the subtropics (due to range contractions). Novel species interactions and warming effects on body size and species survival are likely to reshuffle temperate fish communities and their competitive interactions.