蛇颈龙亚目（Plesiosauroidea）是一群肉食性水生爬行动物，大部分是海生。牠们的化石被发现后，在某种程度上被幻想为 「一个穿过龟壳的蛇」，但其实牠们没有壳。在一般用语里，蛇颈龙类（Plesiosaur）有两种范围，一为蛇颈龙目，另一为蛇颈龙亚目。短颈、大头的蛇颈龙类称为上龙亚目，是中生代的顶级掠食动物；蛇颈龙亚目同时包含长颈（薄板龙科）与短颈（双臼椎龙科）。蛇颈龙类的种类很多，但并非全部如薄板龙般巨大。

蛇颈龙类首次出现于早侏㑩纪，并且繁盛，直到白垩纪-第三纪灭绝事件而灭绝。虽然牠们与恐龙同为中生代爬行动物，牠们并非恐龙。

The Plesiosauria (/ˌpliːsiəˈsɔːriə, -zi-/; Greek: πλησίος, plesios, meaning "near to" and Sauria) or plesiosaurs are an order or clade of Mesozoic marine reptiles (marine Sauropsida), belonging to the Sauropterygia.

Plesiosaurs first appeared in the latest Triassic Period, possibly in the Rhaetian stage, about 205 million years ago. They became especially common during the Jurassic Period, thriving until their disappearance due to the Cretaceous–Paleogene extinction event at the end of the Cretaceous Period, about 66 million years ago. They had a worldwide oceanic distribution.