Sea urchin (Mesocentrotus nudus) is an economically important mariculture species in China and the gonads are the solely edible parts to human. The molecular mechanisms of gonad development have attracted increasing attention in recent years. Although the nanos2 gene has been identified as a germ cell marker in several invertebrates， little is known about nanos2 in adult sea urchins. Hereinto， we report the characterization of Mnnano2， an M. nudus nanos2 homology gene. Mnnanos2 is a maternal factor and can be detected continuously during embryogenesis and early ontogeny. Real-time quantitative PCR (RT-qPCR) and section in situ hybridization (ISH) analysis revealed a dynamic and sexually dimorphic expression pattern of Mnnano2 in the gonads. Its expression reached the maximal level at Stage 2 along with the gonad development in both ovary and testis. In the ovary， Mnnanos2 is specifically expressed in germ cells. In contrast， Mnnanos2 is expressed in both nutritive phagocytes (NP) cells and male germ cells in testis. Moreover， knocking down of Mnnanos2 by means of RNA interference (RNAi) reduced nanos2 and boule expression but conversely increased the expression of foxl2. Therefore， our data suggest that Mnnanos2 may serve as a female germ cell marker during gametogenesis and provide chances to uncover its function in adult sea urchin.