The lamprey genus Lethenteron Creaser & Hubbs, 1922 is widespread across Eurasia and North America, but the number and distribution of its constituent species is not firmly established. After a morphological examination of extant type material of the currently recognized species and their synonyms, Lethenteron mitsukurii (Hatta, 1901) is resurrected with Le. matsubarai Vladykov & Kott, 1978 as its junior synonym. Amongst nonparasitic species Le. reissneri (Dybowski, 1869) and Le. mitsukurii are confirmed as present in Japan and the former is also present on Sakhalin. An in-depth study of large samples of nonparasitic lamprey adults from Japan and Sakhalin Island is needed to determine whether the lower trunk myomere (< 66) individuals from these areas represent one or more undescribed species, or Le. mitsukurii , or Le. reissneri , or a mixture of these three alternatives. The material from the Anadyr Estuary identified by Berg (1931, 1948) as Lampetra japonica kessleri has been re-identified as Le. camtschaticum and there is no evidence that Le. kessleri occurs there. Lethenteron reissneri is reported from the Angara River system, Yenisei River drainage, Russia. Lethenteron alaskense Vladykov & Kott, 1978 is provisionally considered to be a junior synonym of Le. kessleri (Anikin, 1905). Petromyzon ernstii Dybowski, 1872, Ammocoetes aureus Bean, 1881, Petromyzon dentex Anikin, 1905, Lampetra mitsukurii major Hatta, 1911, and Lampetra japonica septentrionalis Berg, 1931 are junior synonyms of Petromyzon marinus camtschaticus Tilesius, 1811. A key is provided to adults of the six species recognized as belonging in the genus Lethenteron .