Along the river–ocean transition, few freshwater species persist into polyhaline zones. Among those insect species capable of living in estuaries, it is unclear which Odonata, Trichoptera, and Coleoptera species can occur at intermittent or average salinities above 18 PSU. During surveys of fish and crustaceans in Prince Edward Island (PEI), we noted 3 unexpected insect species from within subtidal-channels. Multiple instars of Coenagrionidae n. det. and Enallagma civile (Familiar Bluet) were collected around marine macroalgae at 5 localities with 6-26 PSU salinities and observed emerging atop macroalgae. Also, collections of the larvae of the caddisfly Limnephilus externus (Limniphilidae), and adult Haliplus cribrarius (Haliplidae) beetles at sites with maxima of 23 and 20 PSU, respectively, suggest rare occurrences of these species within estuarine ecotones.