Cynthiacetus is an extinct genus of basilosaurid early whale that lived during the Upper Eocene (Bartonian-Priabonian, 40.4 to 33.9 million years ago.) Specimens has been found in the south-eastern United States, Egypt and Peru. The skull of Cynthiacetus was similar in size and morphology to that of Basilosaurus, but Cynthiacetus lacked the elongated vertebrae of Basilosaurus. Uhen 2005 erected the genus to avoid the nomen dubium Pontogeneus (which was based on poorly described and now vanished specimens). Cynthiacetus was smaller than Masracetus.