The West Indian manatee (Trichechus manatus) populations in Florida (T. m. latirostris) and Puerto Rico (T. m. manatus) are considered distinct subspecies and are listed together as endangered under the United States Endangered Species Act. Sustained management and conservation efforts for the Florida subspecies have led to the suggested reclassification of the species to a threatened or delisted status. However, the two populations are geographically distant, morphologically distinct, and habitat degradation and boat strikes continue to threaten the Puerto Rico population. Here, 15 microsatellite markers and mitochondrial control region sequences were used to determine the relatedness of the two populations and investigate the genetic diversity and phylogeographic organization of the Puerto Rico population. Highly divergent allele frequencies were identified between Florida and Puerto Rico using microsatellite (F _ST = 0.16; R _ST = 0.12 (P < 0.001)) and mitochondrial (F _ST = 0.66; Φ _ST = 0.50 (P < 0.001)) DNA. Microsatellite Bayesian cluster analyses detected two populations (K = 2) and no admixture or recent migrants between Florida (q = 0.99) and Puerto Rico (q = 0.98). The microsatellite genetic diversity values in Puerto Rico (H_E = 0.45; N_A = 3.9), were similar, but lower than those previously identified in Florida (H_E = 0.48, N_A = 4.8). Within Puerto Rico, the mitochondrial genetic diversity values (π = 0.001; h = 0.49) were slightly lower than those previously reported (π = 0.002; h = 0.54) and strong phylogeographic structure was identified (F _{ST global} = 0.82; Φ _{ST global} = 0.78 (P < 0.001)). The genetic division with Florida, low diversity, small population size (N = 250), and distinct threats and habitat emphasize the need for separate protections in Puerto Rico. Conservation efforts including threat mitigation, migration corridors, and protection of subpopulations could lead to improved genetic variation in the endangered Puerto Rico manatee population.