Rare plant reintroductions are designed to increase the number of individuals in the wild, but can also improve genetic diversity of populations, reducing both short-term and long-term extinction risks. We used microsatellites developed for the genus Pseudophoenix H. Wendl. ex Sarg. to determine how reintroduced plants of the endangered Pseudophoenix sargentii H. Wendl. ex Sarg. planted in the Florida Keys in the early 1990s contributes to the population genetic structure of the species. We sampled 108 individuals representing wild and reintroduced populations in the Florida Keys and from the ex situ collection at Fairchild Tropical Botanic Garden. The wild stand on Elliott Key and the reintroduced individuals on Long Key displayed evidence of genetic drift, inbreeding, and decreased gene flow. In contrast, the ex situ plants and reintroduced individuals on Elliott Key displayed low inbreeding and higher heterozygosity. All populations deviated significantly from Hardy-Weinberg equilibrium. All pairwise F_ST and G_ST values were significant except comparisons between certain age classes on Elliott Key. Analysis of molecular variance partitioned 87.8% of the genetic variation within populations. Overall, reintroduced plants have contributed to greater heterozygosity of the stand on Elliot Key. Finally, our work shows that the ex situ collection includes wild offspring of individuals that are now extirpated, supporting the ongoing importance of ex situ collections in botanic gardens.