Despite first being described from Virginia, the widely distributed brackish water hydrozoan Blackfordia virginica is often hypothesized to have been introduced from the Black Sea to the United States. However, the alternative view that B. virginica was introduced to the Black Sea also persists in the literature. This study investigates the population structure of B. virginica in the United States to assess the directionality and/or the number of introduction events. During 2009 and 2010, estuaries were sampled from Delaware to Louisiana for brackish water hydromedusae. Nineteen samples of Blackfordia virginica were collected from four localities, including a channel running between St. Catherines Island and Lake Pontchartrain, Louisiana, a region for which it had not been reported prior to this study. We PCR amplified and sequenced two mitochondrial markers (COI & 16S), and one nuclear marker (ITS1). We compared data from individuals collected on the east coast of the United States with individuals collected in California. This revealed low diversity (two haplotypes with a maximal p-difference of 0.03% for COI and just a single haplotype for 16S) and no unique haplotypes at any locality. Low genetic variability, shared haplotypes in disparate localities, and a lack of unique haplotypes in any population are consistent with a founder effect, suggesting a single introduction and subsequent spread throughout the United States.