After teeth, mandibles and mandibular fragments are the best represented element in the early hominin fossil record. Among these, the mandibular hypodigm of Paranthropus boisei stands out as one of the largest ascribed to a single early hominin species, comprising 39 published fossil specimens. Fossils of P. boisei originate from eight localities spread over 1,800km in a North-South axis across eastern Africa—from the site of Malema in Malawi to that of Konso in Ethiopia. However, the vast majority of the remains originate from Koobi Fora in northern Kenya. Most of the Koobi Fora hominins were discovered during the first decade of exploration of the site (1969–1979), and (besides those singularly important fossils that were published in scientific journals) were described in a ma-jor monograph by B. Wood in 1991. Some of the fossils discovered since have yet to be described and analyzed. Here, we describe two previously unpublished hominin mandibles, KNM-ER 42709 and KNM-ER 42801, found by the Koobi Fora Research Project led by Meave and Louise Leakey. Both fossils show mandibular and dental dimensions, as well as autapomorphic traits, typical of P. boisei. We explore quantitatively the position of these two fossils within the diversity of P. boisei mandibles and discuss their implications for the evolutionary history of the species.