Intra- and interspecific competition, two density-dependent processes, are the two main driving forces suggested to promote habitat-based resource polymorphism, but very few studies have provided empirical support for this hypothesis. Furthermore, external drivers like temperature, an important environmental constraint in ectotherms, are often omitted when studying density-dependent habitat selection. Specifically, ambient temperature may have important effects on habitat-based resource polymorphism in ectotherms. Using mixed-effects modelling and isodar analyses, we quantified the effects of water temperature as well as intra- and interspecific competition on abundance and density-dependent habitat selection of brook charr (Salvelinus fontinalis) between littoral and pelagic habitats in 27 Canadian Shield lakes. We found that interspecific competition with white sucker (Catostomus commersonii) reduced brook charr abundances in both habitats but did not affect their density-dependent habitat selection. In contrast, the isodar analysis showed density-dependent habitat selection, confirming that intraspecific competition is a driver of brook charr habitat use in this system. Furthermore, brook charr preferred the littoral habitat at water temperatures below 20.8°C, but preferred the pelagic habitat above this threshold and no longer used the littoral habitat when its temperature was above ~22.2°C. By incorporating an external driver in the density-dependent habitat selection model, we show how the littoral temperature can shape brook charr distribution patterns between the littoral and pelagic habitats by acting as a thermal barrier and thus limiting the willingness of fish to use resources in the littoral habitat. This result suggests that global warming could constrain the diversifying effect of intraspecific competition and prevent resource polymorphism, a phenomenon that promotes adaptive radiation and speciation.