甲龙亚目（学名：Ankylosauria，意为“僵硬蜥蜴”）是属于鸟臀目的一个演化支。甲龙亚目包含了大部分有着骨鳞片形式装甲的恐龙。甲龙亚目都是有着短而壮的腿的笨重四足动物。甲龙亚目已知最早出现在早侏罗纪的中国，直到在白垩纪灭绝。除非洲外，各个大陆都有甲龙亚目的踪迹。第一个发现于南极洲的甲龙亚目为属于甲龙科的南极甲龙属，南极甲龙在1986年发现于罗斯岛。

甲龙亚目是由亨利·费尔费尔德·奥斯本于1923年所命名。在林奈氏分类系统里，这群恐龙常被认为是一个亚目或下目。甲龙下目常与剑龙亚目一起归类于装甲类里，剑龙下目是另一群因为牠们的骨板与尖刺而著名的装甲恐龙。

Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are first known to have appeared in the early Jurassic Period of China, and persisted until the end of the Cretaceous Period. They have been found on every continent except Africa. The first dinosaur ever discovered in Antarctica was the ankylosaurian Antarctopelta, fossils of which were recovered from Ross Island in 1986.