Sessile invertebrates are common invaders of estuarine ecosystems. To expand their non-native ranges, these invasive taxa must contend with the geographically and ecologically discontinuous nature of estuarine habitats, in many cases without the benefit of highly dispersive larval phases. In addition, their population dynamics may reflect contributions from both sexual and asexual reproduction. Here we use genetic methods to explore the population structure of Nematostella vectensis, a dispersal-limited salt marsh anemone, along the Pacific coast of North America. Analysis of 9 highly polymorphic microsatellite loci reveals that asexual reproduction is critical to both maintenance of local population density and regional population expansion. While high levels of genetic differentiation among populations (F ST > 0.3) reflect general restrictions to natural dispersal, the observation of 2 clones distributed across multiple, widely separated sampling sites indicates that long distance dispersal of adult anemones is a major contributor to the spread of N. vectensis. We argue that anthropogenic transport represents the most likely mechanism driving this spread, although rafting may offer an alternative explanation for the observed distribution.