Abstract: During an investigation of myxosporean diversity in the Xiang River (Zhuzhou section, Hunan Province, China), a novel myxosporean species was discovered parasitizing the gallbladder of mandarin fish (Siniperca chuatsi), with no distinct pathological signs observed in the host. Integrated morphological, biological, and molecular analyses confirmed this parasite as a new species of Myxidium, designated as Myxidium chuatsis n. sp., named after its host species. Mature spores of Myxidium chuatsis n. sp. exhibit a fusiform shape in valvular view, featuring sharply pointed ends and 6—8 continuous sutural ridges. In sutural view, spores are oval with a curved suture line. Mature spores measured 10.7±0.4 (10.0—11.4) μm in length, 5.7±0.3 (5.4—6.1) μm in width, and 5.8±0.3 (5.4—6.1) μm in thickness. Two equal-sized pyriform polar capsules are located at the spore extremities, measuring 3.2±0.2 (2.9—3.7) μm in length and 2.8±0.1 (2.6—3.2) μm in width. The distance between polar capsules is 3.7±0.7 (2.8—5.2) μm, with polar filaments coiled 4–5 turns. Morphological comparisons distinguish Myxidium chuatsis n. sp. from all described Myxidium species. BLAST analysis of SSU rDNA sequences revealed the highest similarity (94.74%) to Myxidium sp. (KP030766) infecting the gallbladder of Labeo horie, yet this value falls significantly below intraspecific sequence similarity thresholds. Phylogenetic reconstruction further supports the consensus non-monophyly of Myxidium and highlights the critical role of parasitic microhabitat (gallbladder vs. other tissues) in shaping the evolutionary trajectory of this genus. Additionally, this study reinforces the taxonomic significance of sutural ridge presence/absence in species differentiation.