胎盘（学名：placenta），又称「胞衣」，是一种只有在雌性哺乳类动物怀孕时或是每一只哺乳类动物还是胎儿时才有的暂时器官，位于子宫内侧的表面。

胎盘由两部分组成。一部分和胚胎在生物学与基因上都相连，另一部分是母体的一部分。

胎盘内层是羊膜囊，羊膜囊包含羊水。胎盘植入于子宫壁，并且从母体的血液获取营养与氧气，排出废物。这个接口也是一个障壁，拦下某些可能会伤害胚胎的物质。但是很多物质是胎盘无法拦截的，像是酒精以及一些抽烟产生的物质。几种病毒也可以穿过胎盘，如德国麻疹。

胎盘还有新陈代谢跟内分泌活动。胎盘会分泌黄体激素，对维持怀孕很重要。也会分泌乳促素，增加母体的血糖与血脂，使得胎儿的营养摄取增加。

The placenta (also known as afterbirth) is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, provide thermo-regulation to the fetus, waste elimination, and gas exchange via the mother's blood supply, fight against internal infection and produce hormones to support pregnancy. The placenta provides oxygen and nutrients to growing babies and removes waste products from the baby's blood. The placenta attaches to the wall of the uterus, and the baby's umbilical cord develops from the placenta. The umbilical cord is what connects the mother and the baby. Placentas are a defining characteristic of placental mammals, but are also found in some non-mammals with varying levels of development. The homology of such structures in various viviparous organisms is debatable and, in invertebrates such as Arthropoda, is analogous at best.