Sea level rise is having major impacts on estuaries due to salinity intrusion. These changes in stress profiles have ripple effects in ecosystems, including altering the invasibility of these wetlands depending on the salt tolerance of the invading species. Alternanthera philoxeroides Mart. (Griseb.) (alligator weed), native to South America and long recognized as one of the world's worst freshwater aquatic weeds, recently invaded tidal wetlands in California's San Francisco Bay–Delta Estuary. Generally considered a freshwater-limited glycophyte, observations suggested this invasive macrophyte may have some degree of salinity tolerance, though its degree of tolerance and capacity to spread with increased salinity intrusion were unknown. In two full-factorial greenhouse experiments, we assessed responses of emergent (soil-rooted) and free-floating growth forms of A. philoxeroides to four salinity concentrations (freshwater to euhaline) at the whole-plant (growth, biomass production and allocation, fitness), physiological, and biochemical levels. We also conducted a third experiment exploring the recovery potential of free-floating A. philoxeroides in freshwater following extended exposure to mesohaline to euhaline aqueous salinity. Although sensitivity of A. philoxeroides to increasing salinity was documented, the survival of both growth forms in the full range of salinity treatments was notable and unexpected. Our results indicate A. philoxeroides is a facultative halophyte well adapted to oligohaline–mesohaline salinity levels. Results also revealed the invasive weed's multiple strategies to survive salinity-induced physiological stress, supporting its survival even at elevated polyhaline to euhaline conditions. The macrophyte expressed functional trait responses spanning stress tolerance, avoidance, and escape strategies that may sustain its spread as estuarine salinity intrusion increases with sea level rise.