Vitamin B₁₂ (VB₁₂) is an essential nutrient and growth cofactor for the majority of organisms. It exerts influence not only on the structure of microbial communities and marine primary productivity but also on the global biogeochemical cycles, thus justifying its designation as a "hard currency" in marine ecosystems. Ammonia-oxidizing archaea (AOA), initially isolated from the ocean in 2005, are distinguished by their chemolithoautotrophic characteristics. Genomic, metabolomic, and culture studies have demonstrated that AOA are among the few microbial groups capable of synthesizing VB₁₂ in the ocean. This capability is crucial for maintaining microbial community stability and biogeochemical functions. This review summarizes the measurement methods and distribution characteristics of VB₁₂ in the ocean and the pathways through which AOA produce VB₁₂. It discusses the importance of AOA in marine VB₁₂ supply and outlines the future research directions for VB₁₂ production by AOA.