Among a wide diversity of sexually reproducing species, male ejaculates coagulate to form what has been termed a copulatory plug. Copulatory plugs promote ejaculate movement and retention and inhibit female remating. Female mice mated to males that cannot form a copulatory plug, because the males lack the protein transglutaminase 4 (TGM4), also have fewer successful pregnancies, a phenomenon that is not simply related to reduced fertilization rates. Here we present evidence for a new function of the copulatory plug: the induction of implantation success in females. Using a modified embryo transfer method to control for sperm count and embryonic development, we found that females are less likely to implant when they copulate with males that cannot form plugs. Surprisingly, this result does not correlate with progesterone levels, an important hormone in implantation. We discuss three models to explain the connection between copulatory plugs and implantation, including the hypothesis that plugs contribute to a threshold amount of stimulation required for females to become favorable to implantation.