Haikouichthys /ˌhaɪkuˈɪkθɪs/ is an extinct genus of craniate (animals with notochords and distinct heads) that lived 518 million years ago, during the Cambrian explosion of multicellular life.

[2] Haikouichthys had a defined skull and other characteristics that have led paleontologists to label it a true craniate, and even to be popularly characterized as one of the earliest fishes.

[3] Some researchers have considered Haikouichthys to be synonymous with the other primitive chordate Myllokunmingia,[4] but subsequent studies led by the British paleontologist Simon Conway Morris identified both genera to be distinct, separate taxa on the basis of different gill arrangement,[5] the absence of branchial rays in Myllokunmingia and the myomeres having a more acute shape in Haikouichthys.

[9] Still, numerous segments (myomeres) with rear directed chevrons in the tail indicate that Haikouichthys was indeed a chordate, and complete dorsal, ventral and caudal fins were also found in its specimens.

[9] The fin radials of Haikouichthys show similarity to those of hagfish and lampreys, and they seem to angle "forward" toward the end thought on the basis of internal structures to be the head.