Identifying the dietary niche breadth and trophic ecology of an organism is key in understanding its functional role within a community. An organism’s dietary niche can shift throughout ontogeny and be influenced by biogeographical factors such as latitude and prey availability. It is especially important to identify and assess these potential changes in the diets of predators, specifically keystone predator species, as their dietary niche can dictate the community structure of an ecosystem. In temperate kelp forests systems, California spiny lobsters Panulirus interruptus are major keystone predator species and are suggested to be important regulators of destructive urchin populations, which subsequently promotes kelp persistence. Several studies have identified size as a potential factor in determining the ability of lobsters to control urchin populations, however, there is a current lack of dietary analysis comparing the subadult and adult stages of P. interruptus across their range. We conducted a study across the Southern California Bight (SCB) to assess the dietary niche of the California spiny lobster throughout ontogeny and across a biogeographic gradient. Using sampled tissues of lobsters, invertebrate prey and algae species, we estimated dietary niche space with stable isotope analysis. Our results show that subadult and adult lobsters have similar isotopic dietary niche breadths across their range. We suggest lobsters of different sizes and across space have similar diets and play a similar trophic role in kelp forests. We also analyzed lobster diet composition to determine major prey species comprising the lobster diet. Contrary to previous work regarding lobster diets, our results indicate urchins at many sites are not the primary prey of lobsters. Instead, we see many species of gastropods contributing to their diet. Even with differences in prey species, isotopically, their niche breadths and trophic position remains the same through ontogeny and across space. As lobsters are a major large bodied predator species of the kelp forest and the focus of a multi million dollar fishery, it is imperative to understand aspects of their biology, impacts on their communities, and their potential vulnerabilities to changes in prey abundance.