Antarctic sea stars can occupy different trophic niches and display different trophic levels, but, while the impacts of their body size and environmental features on their trophic niches are potentially important, they are presently understudied. Here we assessed the trophic ecology in relation to the size and habitat of sea stars in a fjord on King George Island (South Shetland Islands) using stable isotope values of carbon (δ13C), nitrogen (δ15N), and sulphur (δ34S). The disc radius influenced δ13C and δ15N values, whereas more limited changes in δ13C or δ34S values were related to arm length. Specifically, δ13C and δ15N values were linked to disc radius in generalist species (Diplasterias brandti and Odontaster validus), which could indicate ontogenetic diet shifts, while this relationship occurred less frequently in more specialised species (Bathybiaster loripes, Notasterias bongraini, and Perknaster sladeni). O. validus had a smaller isotopic niche size in the inner than the outer fjord. The niche overlap between D. brandti and O. validus was low in the inner fjord. Low resource availability within the fjord, linked to higher turbidity, could induce trophic niche constriction and interspecific resource segregation. This could represent a mechanism for competition avoidance in a resource-limited system. Conversely, higher resource availability could allow O. validus to expand and share its isotopic niche with D. brandti in the outer fjord with a limited risk of competition. This trophic plasticity will likely influence how O. validus copes with the present and future modification of environmental conditions induced by climate change.