In this study, the length-weight relationship (LWR) was determined for the marine gastropod and commercial fisheries species, Kellet's whelk (Kelletia kelletii), and possible site- and sex-based differences in the LWR were assessed. Data were collected from 761 Kellet's whelks (1.6–15.5cm shell length, 0.51–404g total weight) that were brought to port by commercial fishermen or collected using SCUBA from three regions across the US geographic distribution of the species: San Diego, Santa Barbara, including the Northern Channel Islands, and Monterey. Model fitting revealed a cubic function to represent the LWR of the entire dataset with strong explanatory power. The fitted model suggested the species demonstrates slightly negative allometric growth, indicating that shell length grows faster than the width of the organism. Whelk sex and collection site did not impact the LWR. The information from this study can be used to inform management decisions for Kellet's whelk conservation, aquaculture, and fisheries harvest and guide future research.