幻龙目（Nothosauroidea）又名孽子龙目，是群三叠纪的海生爬行动物，属于鳍龙超目，可能类似现代的海豹，在水中捕抓食物而回到岸边的岩石与海滩。牠们的身长平均约3公尺，有长的身体与尾巴。脚掌已演化成类似桨状，可在游泳时协助牠们前进。颈部相当长，头部长而平坦，与身体相比头部相当小。长颌部的边缘上有大量、往外突出的锐利牙齿，显示牠们以鱼类为食。

幻龙目由两个亚目构成。肿肋龙亚目，较小、原始的物种。幻龙亚目，包含：色雷斯龙、鸥龙、幻龙，从肿肋龙类演化而来。

类似幻龙类的爬行动物，可能是更完全海生的蛇颈龙类的祖先，蛇颈龙类在三叠纪末取代幻龙类。

Nothosaurs (order Nothosauroidea) were Triassic marine sauropterygian reptiles that may have lived like seals of today, catching food in water but coming ashore on rocks and beaches. They averaged about 3 metres (10 ft) in length, with a long body and tail. The feet were paddle-like, and are known to have been webbed in life, to help power the animal when swimming. The neck was quite long, and the head was elongated and flattened, and relatively small in relation to the body. The margins of the long jaws were equipped with numerous sharp outward-pointing teeth, indicating a diet of fish and squid.