IN BRIEF: Melatonin plays a crucial role in enhancing reproductive performance in small ruminants. This paper reveals the effects of exogenous melatonin on the placental and endometrial rearrangement in early pregnancy in sheep. ABSTRACT: Early pregnancy losses cause 25% of pregnancy failures in small ruminants because of asynchrony between conceptus and uterine signals. In this context, melatonin plays a crucial role in sheep reproductive dynamics, but little is known about its effects during the peri-implantation period. We hypothesized that melatonin supports embryo implantation by modulating the uterine microenvironment. This study aimed to assess the effects of exogenous melatonin on the endometrial and early placental rearrangement. Ten multiparous ewes either did (MEL, n = 5) or did not (CTR, n = 5) receive a subcutaneous melatonin implant (18 mg) 50 days before a synchronized mating. On day 21 of pregnancy, the sheep were euthanized. MEL ewes exhibited a higher prolificity rate (2.8 vs 2.0 embryos/ewe) and plasma progesterone levels (3.84 vs 2.96 ng/mL, P < 0.05) than did CTR ewes. Groups did not differ significantly in embryo crown-rump length. MEL placentas had significantly (P < 0.001) more binucleated trophoblast cells in the chorion region, and ovine placental lactogen expression was significantly (P < 0.05) more strongly upregulated than in CTR. Exogenous melatonin increased significantly (P < 0.05) gene expression of angiogenic factors (VEGFA, VEGFR1, IGF1R), IFNAR2, and PR in the caruncular endometrium. Expression of the MT2 receptor in the endometrium and placenta was significantly (P < 0.05) higher in the MEL group. These results indicate that melatonin implants acted differentially on uterine and placental rearrangement. Melatonin increases differentiation in the placenta and induces changes that could promote vessel maturation in the endometrium, suggesting that it enhances the uterine microenvironment in the early stage of pregnancy in sheep.