Dmrt1 is one of the most important members of the Dmrt family and plays an important role in germ cell differentiation and sex regulation. To explore its regulatory expression mode in the sex differentiation of the scallop Patinopecten yessoensis, we analyzed the sequence characteristics of P. yessoensis Dmrt1 using bioinformatics methods. The expression of Dmrt1 in different tissues was detected using semi-quantitative reverse-transcription PCR (RT-PCR). Quantitative fluorescence RT-PCR (qRT-PCR) and in situ hybridization were used to reveal temporal and spatial expression changes of Dmrt1 in the four stages of gonadal development (proliferation stage, growing stage, mature stage, emission stage). The results showed the following: the sequence of P. yessoensis Dmrt1 contains the DM domain shared by Dmrt family; P. yessoensis Dmrt1 sequence had high homology with Pecten maximus, compared with multiple sequences of other known species; the positive signal detected by in situ hybridization, was mainly located in the cytoplasm of germ cells; and qRT-PCR showed that the expression level of Dmrt1 was significantly higher in the testis than in the ovaries during the growing and mature stages. However, there was no significant change in the ovarian development cycle and the expression level of Dmrt1 remained low. In addition, Dmrt1 transcripts were weakly expressed in the mantle, gill, kidney, and adductor muscle but not in the hepatopancreas. The results of the study showed that the expression characteristics of Dmrt1 in gonadal and sex differentiation of P. yessoensis are generally similar to those of other animals. We speculated that Dmrt1 is a key gene for the regulation of sex differentiation in P. yessoensis.