Horopeta umarere is a new genus and species of extinct baleen whales from the Kokoamu Greensand (early Chattian, Oligocene, in the range 25–27 Ma), Hakataramea Valley, New Zealand. The geological age makes Horopeta umarere one of the earliest named baleen whales. Phylogenetic analysis indicates that Horopeta umarere may be the earliest crown Mysticeti (the sister taxon to Cetotheriidae), or the sister species to the crown Mysticeti; it is clearly not a species of Eomysticetidae. Estimated skull and body length of Horopeta umarere are 1.5-1.6 m and 6.5-7.5 m, respectively. Horopeta umarere shows some features that are linked to gulp feeding as seen in living humpback and rorquals: laterally bowed and robust mandible, D-shaped to teardrop-shaped mandible in cross-section, and posterolaterally deflected triangular coronoid process of the mandible. The sternum of Horopeta umarere is elongate, rod-shaped, and dorsoventrally stout with bilateral anterior and posterior rugose protrusions, indicating the presence of at least two pairs of ribs or costal cartilages. The structure of the skull and mandible are consistent with the use of gulp feeding, but the sternal morphology and rib attachments suggest an early evolutionary stage in gulp feeding employment, where more complex rib attachment may restrict the volume of water and food taken in one gulp compared to living humpback and rorquals. Thus, the morphology of Horopeta umarere has implications for the emergence of gulp feeding in baleen whale evolution as well as the emergence of the crown Mysticeti.