The identification of the sex and sexual maturity of geoducks (Panopea japonica) is difficult because the outer shape of the males and females is indistinguishable. Moreover, depending on their size or age, the annual maturation cycle can vary. Vitellogenin (VTG) is a precursor protein of egg yolk. It has been studied as a biomarker for sex identification in geoducks. This article reports on the cDNA cloning of the P. japonica VTG gene for the VTG fragment. It was found to exhibit homology to known VTGs isolated from other marine molluscs, with more than 50% homology with oyster VTG. An enzyme-linked immunosorbent assay was developed to quantitatively detect the VTG level in geoducks. The VTG polyclonal antibody could recognize the extracted VTG in females, but the level of VTG in males was undetectable, providing a basis for an immunological assay for the identification of the sex and female maturational status of geoducks. Thus, this study established a simple and quantitative technique for the rapid identification of sex and/or maturation of females by using VTG in the hemolymph as a biomarker for the sampled geoducks without dissection or seriously injuring them.