Progesterone is essential in all species for the maintenance of pregnancy, and its withdrawal is required to activate the myometrium and to initiate labor. However, unlike most other species, progesterone levels do not fall at term in humans, raising the paradox as to how labor can occur under the continued influence of progesterone. We hypothesized that an endogenous (myometrial) repressor of the progesterone receptor (PR) could induce a functional withdrawal of progesterone and hence lead to the initiation of labor. We used the human PR as bait in a protein pull-down assay and identified polypyrimidine tract-binding protein-associated splicing factor (PSF) as a PR-interacting protein. PSF functions as a potent inhibitor of PR (but not estrogen receptor) transcriptional activity in mammalian cells. It acts through two novel mechanisms, inducing degradation of the PR through the proteasomal pathway and also interfering with binding of PR to its DNA response element. Importantly, in vivo studies in rats demonstrated a dramatic increase in myometrial PSF expression at term that was temporally associated with reduced levels of the myometrial PR. Accordingly, we propose that PSF acts as a PR corepressor and contributes to the functional withdrawal of progesterone and the initiation of human labor.