The multi-valent manganese is ubiquitous in various marine environments and is involved in many biogeochemical processes with electron transfers between different oxidation states. The involvement of Mn (IV) in the anaerobic oxidation of methane (AOM) in cold seeps has long been proposed by experimental simulation and was suggested to serve as an important sink of methane in the paleo-oceanic environment. Nonetheless, metal-dependent AOM processes still lack explicit geological evidence. Meanwhile, research on micronodules gained momentum in recent years, but the authigenic manganese micronodules in active cold seeps were yet not well studied. In this study, Mn-rich sediments were discovered at the active Haima cold seeps area in the South China Sea, from which Mn-micronodules were then extracted. These micronodules are irregular or subspheroidal with rough and porous surfaces and are mainly composed of 7 Å and 10 Å phyllomanganates. They are enriched in Mn, and deficient in Fe, Co, Ni, Cu, and REYs. Characteristic microbial-like mineralization structures are also observed. Based on these mineralogical/geochemical features and the abnormally high growth rate (calculated by the cobalt chronometer algorithm), these authigenic manganese micronodules were probably formed rapidly via biomineralization and chemical precipitation and were supplied with sufficient Mn by the metal-rich seepage fluids. The occurrence of these micronodules demonstrates manganese bacterial activities, and indirectly reflects metal-dependent AOM. Key Points Authigenic Mn-micronodules are found in the Mn-rich sediments from the active Haima cold seeps of the South China Sea These micronodules are precipitated rapidly from the seepage fluids and formed via microbial-mediated and chemical precipitation The cold seeps are sites where Mn-bacteria thrive and Mn-dependent anaerobic oxidation of methane occurs Plain Language Summary Cold seeps have received extensive attention due to their close association with gas hydrate, petroleum, seepage biological communities, and the global carbon cycle. Sulfate reduction (SR) and anaerobic oxidation of methane (AOM) are important redox processes in seepages. Sulfate has long been regarded as the main oxidant of AOM, while manganese oxides were proposed to involve in AOM a decade ago. Manganese, one of the most abundant transition metals in the Earth's crust, can precipitate and accumulate to form aggregations, like micronodules, in various environments. With the development of analysis methods, micronodules research gain momentum in recent years, but authigenic micronodules from cold seeps have not been well studied yet. In this paper, abundant Mn-micronodules were identified in the surface sediments from the active Haima cold seeps of the South China Sea. Comprehensive geochemical and mineralogical analyses have been conducted on these micronodules to characterize their chemical and mineralogical features (e.g., enriched in Mn, deficient in Fe, Co, Ni, Cu, and REYs, and microbial-like mineralization structures). Based on these characteristics, the occurrence of these micronodules demonstrates manganese bacterial activities, and indirectly reflects metal-dependent AOM. Overall, this study deepens the understanding of authigenic Mn minerals and the Mn geochemistry in active seepages.