A trigonid of a lower molar of a primitive, large hippopotamus from the upper lower Miocene of Mfwangano Island in southwestern Kenya is described. The molar trigonid is similar in size to that of living hippopotamuses, and is comparable in morphology to that of kenyapotamine hippopotamids (Mammalia, Cetartiodactyla) in having a brachyodont crown, bunodont cusps, an M-like structure on the distal trigonid wall, a single-ridged premetacristid, and a buccolingually bifurcate mesial root, and in lacking a paraconid. On the basis of its size and morphology, the specimen appears to be assignable to Kulutherium, which is a putative kenyapotamine previously known from the upper lower Miocene of Kenya and is so far represented only by the upper dentition. The present specimen provides additional evidence that a hippopotamus-sized, large hippopotamid was already living during the early Miocene. If it proves to be Kulutherium, it provides additional evidence that Kulutherium should be assigned to the Kenyapotaminae.