Protected areas are one of the hammers in conservation toolkits, yet few protected areas exist that were designed to protect freshwater ecosystems. This is problematic as freshwater ecosystems are among the most threatened systems on earth. Nonetheless, terrestrial protected areas (TPAs) may afford spill-over benefits to freshwater ecosystems included within their boundaries, but evaluations of these potential benefits for the protection of freshwater fish diversity are lacking. Using fish community data from 175 lakes inside, outside, or bordering TPAs in Ontario, Canada, we sought to determine if TPAs preserve fish functional diversity. We focused on functional diversity because previous work indicated no taxonomic differences between these lakes, but a difference in normalised-length size-spectra slopes inside versus outside TPAs (indicator of unique predator-prey ratios and trophic energy transfer). We expected that communities inside TPAs would show greater functional diversity (i.e. functional dispersion and functional richness) and have more extreme trait combinations (i.e. functional divergence) than communities outside or bordering TPAs. We also tested for differences in the rarity of species-specific functional traits between fish communities inside, outside, or bordering TPAs, between thermal guilds, and across average body size and overall prevalence of the species. Our results indicated no significant differences in functional diversity among lake fish communities inside, outside, or bordering TPAs. However, fish communities inside TPAs had more extreme trait combinations than outside TPAs because abundant species in lake communities outside TPAs had more ubiquitous trait combinations than abundant fishes inside TPAs. Small-bodied species showed greater functional rarity than large-bodied species, indicating that small-bodied fishes fill functionally unique roles while the most prevalent, large-bodied species possess a more generalist set of traits. Overall, the similarity of functional diversity metrics for lake fish communities inside, outside, or bordering TPAs in Ontario suggests that TPAs capture the functional diversity of Ontario's lake fish communities. However, we encourage similar evaluations in regions where environmental conditions and stressors are more distinct across TPA boundaries than they are in Ontario, as these types of evaluations will inform guidelines for the design of freshwater protected areas and monitoring of their effectiveness in the future.