<p>Dietary specialization is a key trait influencing species resilience in changing environments, yet it is often inferred as the fundamental niche—an inherent species feature—rather than the realized niche, which reflects actual resource use under specific ecological conditions and can vary across spatial, temporal and biological organizational scales (individuals to populations). In Antarctic ecosystems undergoing rapid changes, understanding trophic plasticity in keystone species is essential. Here, the dietary composition and specialization of the Antarctic sea urchin <i>Sterechinus neumayeri</i> were assessed across spatial, temporal and organizational scales along the Western Antarctic Peninsula (WAP, 64–67°S; 61–66°W). Samples were collected at thirteen sites spread over two expeditions conducted in February–March 2023 (southern WAP) and 2024 (northern WAP), spanning a north–south gradient along the WAP and providing spatial coverage across the region. Temporal variability was inferred from complementary dietary tracers applied to the same individuals: gut content metabarcoding (COI gene tag), providing short-term, high-resolution taxonomic information, and stable isotope analysis (δ¹³C, δ¹⁵N, δ³⁴S), offering a longer-term view of assimilated diet at lower resolution. Results revealed significant spatial variation in dietary composition and specialization. Although diatoms dominated diet across sites, contributions of macroalgae and invertebrates varied. Alpha and beta prey diversity and isotopic niche volumes differed significantly among sites, and correspondence between short-term and longer-term dietary patterns was weak. Finally, population-level averages masked substantial individual dietary specialization within and between sites. These findings indicate that <i>Sterechinus neumayeri</i> exhibits context-dependent dietary specialization, emphasizing the importance of multi-scale approaches in marine ecosystems.</p>