After the deployment of several whale carcasses in Monterey Bay, California, a time-series analysis revealed the presence of a new species of Osedax, a genus of bone-eating siboglinid annelids. That species is described here as Osedax roseus n. sp. It is the fifth species described since the erection of this genus and, like its congeners, uses a ramifying network of "roots" to house symbiotic bacteria. In less than 2 months, Osedax roseus n. sp. colonized the exposed bones of a whale carcass deposited at 1018-m depth, and many of the females were fecund in about 3 months post-deployment. As with other Osedax spp., the females have dwarf males in their tube lumens. The males accrue over time until the sex ratio is markedly male-biased. This pattern of initial female settlement followed by gradual male accumulation is consistent with the hypothesis that male sex may be environmentally determined in Osedax. Of the previously described species in this genus, Osedax roseus n. sp. is most similar to O. rubiplumus, but it has several anatomical differences, as well as much smaller females, dwarf males, and eggs. Osedax roseus n. sp. is markedly divergent (minimally 16.6%) for mitochondrial cytochrome oxidase subunit I (mtCOI) sequences from any other Osedax species.