We investigated genetic diversity and structure of urban white-footed mouse, Peromyscus leucopus, populations in New York City (NYC) using variation at 18 microsatellite loci. White-footed mice are "urban adapters" that occur at higher population densities as habitat fragments are reduced in area, but have a limited ability to disperse through urbanized areas. We hypothesized that this combination of traits has produced substantial genetic structure but minimal loss of genetic variation over the last century in NYC. Allelic diversity and heterozygosity in fourteen NYC populations were high, and nearly all of our NYC study sites contained genetically distinct populations of white-footed mice as measured by pairwise FST, assignment tests, and Bayesian clustering analyses performed by Structure and BAPS. Analysis of molecular variance revealed that genetic differences between populations separated by a few km are more significant than differences between prehistorically isolated landmasses (i.e. Bronx, Queens, and Manhattan). Allele size permutation tests and lack of isolation-by-distance indicated that mutation and migration are less important than drift as explanations for structure in urban, fragmented P. leucopus populations. Peromyscus often exhibit little genetic structure over even regional scales, prompting us to conclude that urbanization is a particularly potent driver of genetic differentiation compared to natural fragmentation.