The Christmas Tree Worm (Spirobranchus sp.) is a biota that lives in association with hermatypic corals. This bioeroder damages the structural integrity of corals, making them susceptible to breakage. Therefore, further investigation is required to determine the distribution, diversity, uniformity, and predominance of CTW on hard corals in the Sempu Island Nature Reserve's waters. The method used is the belt transect method, which refers to the reef check. CTW species will be identified based on their operculum, and corals will be identified based on their growth form, genus, and species. The data gathered were processed, and the abundance value and ecological index were obtained. The CTW species found at the five stations were Spirobranchus gardineri, Spirobranchus cruciger, and Spirobranchus spp., associated with massive corals of the genus Porites and genus Goniastrea. Water quality factors such as currents, sedimentation, and nitrates affect CTW's presence. Medium-to-fast currents circulate around Sempu Island, where high sedimentation rates of 73.73 mg/cm2/day decrease light penetration into the waterways. Additionally, elevated nitrate levels contribute to the growth of CTW operculum-covering turf algae. CTW abundance values ranged from 0.018 to 0.084 individuals/m2, diversity index (H') ranged from 0.403 to 0.760 (low), and uniformity index (E) ranged from 0.582 to 0.966 (unstable to stable). The dominance index (C) values ranged from 0.513 to 0.705 (medium). Based on the index value, it was found that the CTW in Sempu Island was in the unstable category, so there was moderate dominance.