Abstract Premise of study: Molecular population genetics is a powerful tool to infer how species responded to past environmental change. In the northern hemisphere, interest is increasing in how species responded to changes in ice coverage and temperature during the last glaciation maximum (LGM, between 18000–21000 yr ago) with a common assumption that glacial refugia were located at the southern edge of a species range. Methods: We reconstructed the glacial and postglacial phylogeography of Sabatia kennedyana, a member of the Atlantic Coastal Plains Flora with a current distribution from Nova Scotia (NS) to South Carolina, using both cpDNA and nuclear markers. We also examined clinal variation in morphological traits, in particular relative investment in asexual vs sexual growth. Key results: We find strong evidence that the species did not reside in southern glacial refugia, but rather in primary glacial refugia off the exposed continental shelf extending from Cape Cod and that this area was responsible for the founding of modern populations across the range from Nova Scotia (NS) to the United States. Additionally, based on the finding of higher cpDNA diversity and older cpDNA lineages in NS, we propose that multiple founder events occurred in NS, while only a single lineage gave rise to current populations in the United States. Conclusions: By understanding how S. kennedyana responded to past shifts in climate and by identifying areas of high genetic diversity in the northern range edge, we discuss the potential response of the species to future climate change scenarios. ; Usage notes Alignment of the spacers between trnS and trnG and rpL20-5’rpS12 in 87 samples from Sabatia kennedyana, representing the northern (NS, Canada), central (MA, USA) and southern (NC, USA) range of the species. The spacers between trnS and trnG and rpL20-5’rpS12 were aligned with a consensus length of 1161 bp which was reduced to 812 bp after removing gaps and missing data. Among the 87 samples, 28 cpDNA haplotypes harbouring 38 ...