Many species endemic to isolated islands are of conservation interest because of concerns over the potentially devastating effects of environmental stochasticity and the pending threat of anthropogenic influences and invasive species. The effective size (N_e) of these species is a key parameter in their conservation because it predicts the detrimental effects of inbreeding or genetic drift and can be used to inform management plans. We used microsatellite allele frequencies (4 loci) and mtDNA (control region; cytochrome b) sequences to assess the genetic diversity and the effective number of breeders (N_b) in the Nihoa Millerbird (Acrocephalus familiaris kingi), a critically endangered passerine endemic to the northwest Hawaiian island of Nihoa. Using samples collected in 2007 and 2009, our results reveal extremely low levels of genetic diversity at both microsatellites and mtDNA, and both approximate Bayesian computation (ABC) and sibship methods indicate that the effective number of breeders (N_b) for this species is between 5 and 13 individuals. Our analysis highlights the utility of ABC and sibship methods for estimating N_e in species with low genetic polymorphism or few loci. We compare our results to a recent genetic study of this species and document the loss of alleles at two microsatellite loci and one unique mtDNA haplotype. We discuss our findings in the context of the planned translocation of Nihoa Millerbirds from Nihoa to Laysan Island.