Dmrt genes encode a large family of transcription factors involved in sexual development. These genes have been well studied in various species. However, their expression profiles and functions in bivalves are still unclear. As an important member of the Dmrt gene family, Dmrt2 is controversial because of its role in sex determination and differentiation. In the current study, pmDmrt2 (Dmrt2 from Pinctada martensii) was screened from the male gonads cDNA library. The full length of pmDmrt2 cDNA is 966 bp, with an open reading frame of 836 bp (58–893), which encodes a peptide of 278 amino acids. This gene shows 36.2%, 35.9%, 34%, 33%, 32.7%, and 21.9% identity to Dmrt2 of zebrafish, clawed frog, chicken, house mouse, human, and sea urchin, respectively. Despite the low sequence identity, the highly conserved double sex and mab-3 domain was predicted to exist in pmDmrt2. Results from the reverse transcription—polymerase chain reaction indicate that pmDmrt2 is transcribed mainly in the male gonad, slightly in the gill, but not in other tissues. The gene is first transcribed in the early male gonads, and peaks in the mature male gonads. During transition from male to female, pmDmrt2 is gradually downregulated until it eventually becomes nonexistent in the mature female gonads. In situ hybridization analysis reveals that pmDmrt2 mRNA is localized specifically in the spermatogonia, spermatocytes, and spermatids in the male gonads. Our investigation indicates that pmDmrt2 might play a functional role during spermatogenic cell differentiation from spermatocytes and spermatids into sperm. Bivalves and mammals use at least several similar mechanisms to control sexual development.