Niche partitioning has long been considered to be a fundamental part of speciation and difference in body size is a straightforward proxy for ecology and niche partitioning. Here we report a new aetiocetid specimen from the upper part of Morawan Formation (Oligocene; late Chattian, ~26–24 Ma), Ashoro, Hokkaido, Japan. This specimen comprises an isolated left squamosal and an isolated left periotic; the morphology of the squamosal is distinguishable from all aetiocetids, but shows a close match with the aetiocetid Morawanocetus yabukii in that: the dorsal margin of the zygomatic process of the squamosal is twisted dorsomedially; the zygomatic process expands dorsoventrally; the zygomatic process is transversely thin; and the anterior tip of the zygomatic process is acutely tapered. The estimated body size of this Morawanocetus-like specimen is ca. 8 m, almost twice the size of all known aetiocetids (3–4 m). Moreover, the preserved periotic reveals structures that are otherwise poorly known in aetiocetids. This large Morawanocetus-like aetiocetid sheds new light on niche partitioning of Oligocene toothed mysticetes and evolution of body size in Mysticeti.