We characterized the morphological variation and algal species associations of the filamentous green alga, Acrosiphonia arcta (distinguishing it from its closely related sister genus Spongomorpha), in two contrasting environments in Nova Scotia, Canada. The habitats differed in seawater salinity and temperature (and fluctuations thereof), substratum, vertical position on the shore, wave exposure, turbidity, and diversity of macroalgae and macrofauna. Acrosiphonia arcta thalli exhibited distinct morphologies in the different environments. Thalli from the open-ocean Whitehead site (Atlantic coast) exhibited the typical A. arcta morphology of profuse, short, curved branches and rhizoids binding the filaments together to form hemispherical to spherical tufts. Thalli from the sheltered Pomquet Harbour Estuary (Gulf of St. Lawrence coast) consisted of matted clumps with extensive rhizoidal growth at the base but little branching otherwise. Acrosiphonia arcta was primarily associated with thick leathery and crustose (coralline) algae at Whitehead. In the Pomquet Harbour Estuary, a less stable habitat than Whitehead, sheet and filamentous algae were in greater abundance with A. arcta than at Whitehead. Damaged apical cells from thalli of both Pomquet and Whitehead A. arcta were able to regenerate new filamentous tips or to give rise to rhizoids, phenomena not previously documented for Acrosiphonia in North America. Morphological variation of A. arcta in different habitats points to its ability to respond to and persist in a wide range of abiotic (and possibly biotic) conditions, and helps to explain its seasonal dominance on the east and west coasts of North America.