Methylmercury (MeHg), a neurotoxic pollutant, accumulates along marine food chains and threatens human health through seafood intake. Tropical coral reefs sustain exceptional marine biodiversity, yet few long-term surveys track MeHg trophic transfer in remote reefs free from intense human interference. This work presents a decade-long (2016 – 2025) investigation of MeHg bioaccumulation and biomagnification in fish at Meiji Reef, a nearly undisturbed oceanic atoll in the Nansha Islands of the southern South China Sea. We collected 255 fish specimens covering 39 species, measuring muscle MeHg contents together with sediment mercury levels and stable isotopes (δ¹³C, δ¹⁵N) to quantify trophic niches and trophic magnification slopes (TMS). Results showed that the δ¹⁵N values ranged from 4.30‰ in the herbivorous to 10.31‰ in the apex carnivore, while δ¹³C values ranged from -19.60‰ to -9.42‰. The total convex hull area of the whole community remained between 34.00 and 42.60 ‰², with interannual convex hull overlap consistently exceeding 0.58 (up to 0.77 between consecutive years), reflecting stable trophic niche occupation across years. In surface sediment, total mercury maintained low background (1.10 – 1.99 ng/g dw) values, whereas MeHg rose slightly (0.03 – 0.12 ng/g dw). In fish, MeHg levels followed a consistent trophic order: carnivores (648.7 ± 69.80 ng/g dw) > omnivores (98.90 ± 11.90 ng/g dw) > herbivores (18.30 ± 1.30 ng/g dw), without notable interannual fluctuations for the whole community. Body weight showed a significant positive correlation with MeHg in carnivorous (slope = 0.38, R² = 0.29, p < 0.001) and herbivorous (slope = 0.23, R² = 0.20, p < 0.001) fishes but not in omnivores (p ＞ 0.05). Decadal TMS values remained stable between 0.37 and 0.46 (TMF: 2.34 – 2.88, R²: 0.67 – 0.89, p < 0.001), with no significant interannual fluctuations detected, indicating that mercury inputs to the study area are largely controlled by natural background processes. In comparison with TMS values from 46 regions worldwide (0.06 – 0.32), the mean TMS in our study area (TMS: 0.39, TMF: 2.47) ranked the highest. Low-trophic fish (TL 2.00 – 2.99) here held far lower MeHg (21.16 ng/g dw) than global counterparts (114.2 ng/g dw, p < 0.001), while high-trophic predators (TL 3.00 – 4.00) showed comparable mercury loads (320.5 ng/g dw) which was significantly higher than the global average (192.6 ng/g dw, p < 0.01), demonstrating exceptionally efficient trophic transfer in this low-background tropical coral reef. This highlights that trophic transfer efficiency, rather than environmental concentration, determines mercury risk in reef ecosystems. This study clarifies the unique biomagnification pattern of MeHg in high-diversity reef food webs and supplies missing long-term observational data for isolated tropical coral reefs, supporting risk evaluation of local reef fish.