A reciprocal communication between the implantation-competent blastocyst and the receptive uterus is essential to successful implantation and pregnancy success. Progesterone (P4) signaling via nuclear progesterone receptor (PR) is absolutely critical for pregnancy initiation and its success in most eutherian mammals. Here we show that a nuclear protein high-mobility group box-1 (HMGB1) plays a critical role in implantation in mice by preserving P4-PR signaling. Conditional deletion of uterine Hmgb1 by a Pgr-Cre driver shows implantation defects accompanied by decreased stromal cell Hoxa10 expression and cell proliferation, two known signatures of inefficient responsiveness of stromal cells to PR signaling in implantation. These mice evoke inflammatory conditions with sustained macrophage accumulation in the stromal compartment on day 4 of pregnancy with elevated levels of macrophage attractants Csf1 and Ccl2. The results are consistent with the failure of exogenous P4 administration to rescue implantation deficiency in the mutant females. These early defects are propagated throughout the course of pregnancy and ultimately result in substantial subfertility. Collectively, the present study provides evidence that nuclear HMGB1 contributes to successful blastocyst implantation by sustaining P4-PR signaling and restricting macrophage accumulation to attenuate harmful inflammatory responses.