The human placenta is essential for pregnancy, yet the dynamic regulation of its DNA methylome during development remains poorly understood. To address this, we generated a single base-resolution DNA methylation atlas of human placental trophoblasts across gestation. We identified a lineage-specific methylation program established immediately after implantation, followed by gestational age-dependent methylation accumulation. This program acted as a developmental barrier that coordinated placental maturation and immune homeostasis. By integrating histone modifications and functional perturbations, we revealed a temporal dynamic between H3K27me3 and DNA methylation that silenced proinflammatory loci and a switch from H3K9me3 to DNA methylation at repeat elements to maintain genome stability. Aberrant methylation in preterm placentas underscored the physiological importance of methylation regulation. Using human trophoblast stem cells, we demonstrated that DNMT1 was critically required for maintaining trophoblast methylome integrity and self-renewal, while the DNMT3 family were equally essential for trophoblast survival. Overall, we established a placenta-specific epigenetic framework for human trophoblast development.