Prostaglandin D2 (PGD2) belongs to the superfamily of ubiquitous signalling molecules, the prostaglandins ; these bind to specific G-coupled transmembrane receptors, inducing various transduction pathways. Prostaglandins PGE2 and PGF2alpha have several identified functions during ovulation, fecondation and embryo implantation. However, the roles of PGD2 within the male or female reproductive organs are still largely unknown, even though the PGD2-producing enzyme, prostaglandin D synthase (PGDS), is detected in these organs. In this study, we summarize recent data highlighting new functions of PGD2 in the onset of testicular embryogenesis and in the growth inhibition of ovarian cancer cells. In both cases, PGD2 acts by activating the function of the Sertoli cell differentiating factor SOX9.