Ship sturgeon (Acipenser nudiventris) is a critically endangered fish that is listed on the International Union for Conservation of Nature’s Red List of Threatened Species. Sixteen individuals from the Chinese section of the Ili River were genotyped using genome re-sequencing technology. By applying a genomic relatedness estimation with 1,527,694 genome-wide SNP markers, we found that the coancestry coefficients showed a high level of relatedness between individuals. The effective population sizes over 500 generations were estimated, and this showed that the effective population sizes began to dramatically decline from about 14,840 to 171 individuals when going back four generations from the current population. Artificial reproduction techniques guided by genomic relatedness may be a valuable approach to the conservation of this critically endangered fish species.