Heribaudiella fluviatilis is a freshwater species in the predominantly marine class of brown algae (Phaeophyceae). The first reported North American population was collected in 1898 from Island Brook, CT. Here we confirm that the species was once present in Island Brook but has been extirpated from that location. Our 2016 survey rediscovered Heribaudiella in the New England flora, in 6 streams in western Connecticut ∼70 km inland from marine water. Ecological data indicate these streams are deeper and have large-grained sediments, but lower specific conductance, dissolved NO₃^-, and inorganic P as soluble reactive phosphorous (SRP) than nearby streams lacking this alga. We ran a multivariate, boosted regression tree (BRT) analysis, which confirmed that the niche of Heribaudiella in Connecticut is limited to minimally disturbed streams with greater pH, a high percentage of streambed boulders, and lower concentrations of dissolved NO₃^- and SRP.