Due to a population bottleneck, northern elephant seals (Mirounga angustirostris ) have very low genetic diversity, making them ideal model organisms for assessing the impact of genetic and non-genetic factors on the gut microbiome. In our study, we were especially interested in the role of sex given the northern elephant seal’s extreme sexual dimorphism. We investigated 54 northern elephant seal pups that were rescued from along the California coastline and brought to The Marine Mammal Center, a rehabilitation facility. Using a metabarcoding approach, we characterized microbial communities shortly after admission to the facility and found that both sex and geographic origin explained microbial variation. We detected significant differences in microbial class and order composition between sexes. We further analyzed paired samples from 24 seals at two time points, shortly after admission to the rehabilitation facility and a month post-acclimation in the facility. Between these two time points, microbial diversity increased, likely due to changes in diet. While there was an overall convergence of microbiome composition in a shared environment over time, remaining differences in microbial composition were explained by sex and host genetics.