eremus Kellogg, 1929 (type) Nannocetus is an extinct baleen whale belonging to the family Cetotheriidae.

[1][2] Nannocetus is a diminutive mysticete measuring 13 feet (4.0 meters) long.

It is characterized by the ventral orientation (in posterior view) of the postglenoid process; postglenoid process twisted medially (in ventral view) relative to the anteroposterior axis of the skull; equal projection of the ventral and dorsal lobes of the tympanic than the dorsal lobe; deeper notch separating the two lobes of the tympanic; reniform morphology of the tympanic in ventral view; lip of the tympanic slightly inflated; sub-rectilinear medial edge of the involucrum with a step in its anterior third; anterior process of the petrosal sub-triangular; thin crista transversa of the petrosal; and pars cochlearis hemispherical.

[3] The holotype is UCMP 26502 was collected from the Late Miocene (Messinian) Towsley Formation of Humphreys, Los Angeles County, California.

[4] It has been traditionally assigned to Cetotheriidae since its description, a classification that still stands and has been supported by recent cladistic analyses of 'cetothere' relationships.