Many marine species have widespread geographic ranges derived from their evolutionary and ecological history particularly their modes of dispersal. Seagrass (marine angiosperm) species have ranges that are unusually widespread, which is not unexpected following recent reviews of reproductive strategies demonstrating the potential for long distance dispersal combined with longevity through clonality. An exemplar of these dual biological features is turtlegrass (Thalassia testudinum) which is an ecologically important species throughout the tropical Atlantic region. Turtlegrass has been documented to have long distance dispersal via floating fruits and also extreme clonality and longevity. We hypothesise that across its range, Thalassia testudinum will have very limited regional population structure due to these characteristics and under typical models of population structure would expect to detect high levels of genetic connectivity. There are very few studies of range-wide genetic connectivity documented for seagrasses or other sessile marine species. This study presents a population genetic data set that represents a geographic area exceeding 14,000 km2. Population genetic diversity was evaluated from 32 Thalassia testudinum populations sampled across the Caribbean and Gulf of Mexico. Genotypes were based on nine microsatellites and haplotypes were based on chloroplast DNA sequences. Very limited phylogeographic signal from cpDNA reduced the potential comparative analyses possible. Multiple analytical clustering approaches on population genetic data revealed two significant genetic partitions: 1) The Caribbean, and 2) The Gulf of Mexico. Genetic diversity was high (HE = 0.641) and Isolation by Distance was significant, gene flow and migration estimates across the entire range were however modest, we suggest that the frequency of successful recruitment across the range is uncommon. Thalassia testudinum maintains genetic diversity across its entire distribution range. The genetic split may be explained by genetic drift during recolonsation from refugia following relatively recent reduction in available habitat such as the last glacial maxima.