Understanding the dietary ecology of endangered species is essential for effective conservation. This study explores the trophic ecology of Hippocampus haema, an endangered seahorse species in South Korea, using stable isotope analysis (SIA). We assessed the feasibility of using the distal tail tip, which is rich in muscle tissue, for SIA to minimize specimen loss. Although this method has not been proven to be less invasive than fin clipping, it offers a practical alternative for small-sized or juvenile seahorses for which fin clipping provides insufficient tissue for analysis. A preliminary analysis confirmed that isotope values were consistent across different tail sections, validating the use of tail tips in dietary studies. We conducted two phases of analysis: (1) seasonal and size-based variation in δ¹³C and δ¹⁵N values using 36 specimens collected across four seasons (2015-2016), and (2) estimation of dietary contributions using a Bayesian mixing model (MixSIAR) based on δ¹³C and δ¹⁵N values of H. haema and six potential prey taxa collected in 2020. Seasonal variation was observed in δ¹⁵N values, with higher values in July, potentially reflecting breeding season dietary shifts. A significant positive correlation between isotopic values and body size suggests size-related changes in prey selection, though trophic position remained stable. MixSIAR results indicated that slower, smaller taxa such as Caprella, Harpacticoida, and Limnoria were the primary prey items, regardless. These findings provide insights into the feeding ecology and habitat reliance of H. haema, guiding conservation strategies to protect the species and its Sargassum-dominated habitat.