古鲸亚目（学名：Archaeoceti）即古代鲸鱼，是现在鲸鱼的祖先。古鲸亚目是鲸目三个亚目之中，唯一已经绝种的。古鲸亚目拥有后肢，不像今日的鲸类。最早的古鲸亚目可能完全在陆地上活动。其灭绝原因至今众说纷纭，其中一种解释是由于气候变化，海洋变得过份寒冷，造成古鲸类灭绝。

Archaeoceti ("ancient whales"), or Zeuglodontes in older literature, is a paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene (55 to 23 million years ago). Representing the earliest cetacean radiation, they include the initial amphibious stages in cetacean evolution and are thus the ancestors of both modern cetacean suborders, Mysticeti and Odontoceti. This initial diversification occurred in the shallow waters that separated India and Asia 53 to 45 mya, resulting in some 30 species adapted to a fully oceanic life; though both echolocation and filter-feeding evolved during a second radiation 36 to 35 mya.