C-Mos, a proto-oncogene, regulates oocyte maturation by activating the classical MAPK pathway in cells. To examine the function ofC-MosinHyriopsis cumingii,C-Moswas identified in this study. The full-length cDNA ofC-Moswas 2213 bp, including 144 bp in the 5′ UTR, 923 bp in 3′ the UTR, and 1146 bp in the open reading frame (ORF) region. During early gonad development, the expression ofC-Mosfrom 4 to 6 months of age inH. cumingiiwas significantly higher than that in other months, with the highest expression in 6-month-oldH. cumingii, suggesting thatC-Mosmay be involved in early gonadal development inH. cumingii. Clear hybridization signals were found by in situ hybridization in the oocytes, oocyte nucleus and oogonium, and a small number of hybridization signals were found in the follicular wall of the male gonads. In addition, theC-MosRNA interference (RNAi) assay results showed that the knockdown ofC-Moscaused a down-regulation ofERKandP90rsk. In summary, these results indicate thatC-Moshas a crucial part to play in gonadal development inH. cumingii.