The western Tianshan, situated on the southwestern margin of the Central Asian Orogenic Belt, underwent a tectonic evolution associated with the South Tianshan Ocean—a branch of the Paleo- Asian Ocean. This region hosts a series of Devonian marine sedimentary manganese deposits. Based on field investigations, this paper examines the mineralogical and geochemical characteristics of four Devonian manganese deposits in the western Tianshan: the Bosuoguoshan manganese deposit, Guoergou Fe- Mn deposit, Kalanggou Mn deposit, and Xiawengxi Fe- Mn deposit, using petrographic observation, electron probe microanalysis (EPMA), and major and trace element geochemical analyses. The research results reveal that the manganese mineralization is mainly composed of oxides (manganosite, pyrolusite, romanèchite, hausmannite, birnessite, braunite, todorokite, cryptomelane, hollandite, and jacobsite), carbonates (rhodochrosite and manganese- bearing calcite), and silicates (rhodonite, spessartine, tephroite, bementite, caryopilite, alleghanyite, and manganoan phlogopite). MnO contents in the manganese oxide minerals range from 30. 9% to 83. 2%. Within the manganese carbonates, rhodochrosite exhibits average molar ratios of MnCO3/(MnCO3+MgCO3+CaCO3)=0. 92 and MgCO3/CaCO3=0. 17. The Bosuoguoshan manganese ores exhibit a depleted rare earth element (REE) pattern, consistent with passive continental margin signatures. In contrast, the Guoergou manganese ores display an enriched REE pattern with prominent Eu and Ce anomalies, characteristic of an active continental margin setting. The lgU- lgTh and Fe/Ti versus Al/(Al+Fe+Mn) diagrams collectively indicate a primary hydrothermal origin for the manganese mineralization. MnO- SiO2 scatter plots reveal that these deposits universally underwent high- energy reworking processes. Combined with the analysis of the regional tectonic evolution, we propose that the Devonian manganese deposits of the western Tianshan formed in a locally extensional environment on a continental margin related to the subduction of the South Tianshan Ocean. The mineralization was primarily sourced from endogenic processes, including submarine hydrothermal activity and volcanism. Small marginal oceanic basins such as Jigen, Kuokesaleling, Tieliemaiti, and Erbin exhibit significant potential for future manganese exploration.