羊膜动物（Amniota）是一群四足脊椎动物，包括合弓类动物（哺乳类与似哺乳爬行动物）与蜥形类（含爬行动物、鸟类）。羊膜动物借由产卵、胎生等方式繁衍，胚胎由多层膜来保护。在真兽类哺乳动物，更演化出羊膜来包覆胎儿。羊膜动物与两栖动物的差异在于保护胚胎的膜，与缺乏幼体变形为成体的阶段。

根据较广泛的定义，第一个羊膜动物是Casineria kiddi，生存于3亿4000万年前，属于爬行形类（Reptiliomorpha），外表类似小型蜥蜴。羊膜动物的蛋可处在远离水的环境，使牠们可迁移入更干燥的环境。羊膜动物的蛋可以交换空气并排放废物，使蛋与羊膜动物往更大尺寸的趋势演化发展。羊膜动物散布于全球，并成为优势陆地动物。羊膜动物主要分为两个演化支。目前已知最古老的合弓类动物是Protoclepsydrops，生存于3亿2000万年前，而最古老的蜥形类动物可能是古窗龙，生存于宾夕法尼亚纪中期，约3亿1200万年前到3亿600万年前。

Amniotes (from Greek ἀμνίον amnion, "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός amnos, "lamb") are a clade of tetrapod vertebrates comprising the reptiles, birds and mammals that lay their eggs on land or retain the fertilized egg within the mother. They are distinguished from the anamniotes (fishes and amphibians), which typically lay their eggs in water. Older sources, particularly prior to the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the discredited idea of the Great Chain of Being.