翼龙（Pelycosaurs）（来自希腊语pelyx，意为 "碗"，sauros，意为 "蜥蜴"）是四足动物的一个目，出现在石炭纪晚期，在二叠纪早期蓬勃发展，然后在二叠纪晚期被其后代therapsids所取代。 这个原始群体是陆地生命的主要形式，时间约为4000万年，是哺乳动物统治时间的五分之三，并且是最早进化的羊膜动物（非两栖四足动物）之一。 它是最原始的四足动物群体之一，在基性四肢动物与早期两栖动物之后。

Pelycosaurs ranged in size from a large rat to a wolf to bigger than an elephant. Being the dominant form of terrestrial life, they evolved to exploit numerous roles, including herbivores, insectivores, and carnivores. Some resembled robust lizards, others were more foreign, such as the iconic Dimetrodon grandis, an apex predator 3 1/2 meters (11 feet) in length, with huge vertebral extensions that created a large skin-covered sail. This animal had a somewhat dragon-like appearance. Other pelycosaurs shared the back sail, which is believed to have been used to increase the temperature of the animal. Another pelycosaur, Cotylorhynchus, was a titanic barrel-shaped herbivore with a small head and a total length exceeding 6 m (20 ft). It would have been longer and heavier than the largest elephants alive today.

Pelycosaurs were among the first amniotes, meaning they had well-developed eggs, allowing them to venture far from the water — amphibians must remain moist and lay their eggs in water. The pelycosaurs were the first successful amniote group, capable of colonizing continental interiors that had been reached by plants and insects tens of millions of years earlier. However, the time when the pelycosaurs were successful, the Early Permian, was also a time of great deserts, as all the world's continents had merged into the supercontinent Pangaea, which included practically all the world's land mass. The interior of Pangaea would've been the biggest desert the world had ever seen. For this reason, art that depicts reconstructions of pelycosaurs often show them in a desert-like background, although many groups of pelycosaurs probably lived in forests.

翼龙偶尔会与恐龙混为一谈，特别是在为儿童简化问题时，但成年人应该知道，恐龙和翼龙之间没有任何关系，只是它们都是地球上一段时期内的主要生命形式。 尽管翼龙偶尔有爬行动物的外表，但它们是半兽人，而不是碟形目（爬行动物），实际上是哺乳动物的祖先。 翼龙的一个群体，therapsids，继续获得非常哺乳动物的特征，经过一亿多年，产生第一个真正的哺乳动物。 所有翼龙在二叠纪结束前就已灭绝，使它们免于结束时的大规模灭绝。