Twenty new species of the millipede genus Chaleponcus Attems, 1914, are described from the Udzungwa Mountains: C. netus sp. nov., C. quasimodo sp. nov., C. malleolus sp. nov., C. scopus sp. nov., C. nikolajscharffi sp. nov., C. mwanihanensis sp. nov., C. basiliscus sp. nov., C. krai sp. nov., C. nectarinia sp. nov., C. circumvallatus sp. nov., C. ibis sp. nov., C. vandenspiegeli sp. nov., C. vilici sp. nov., C. teres sp. nov., C. hamerae sp. nov., C. termini sp. nov., C. gracilior sp. nov., C. mwabvui sp. nov., C. howelli sp. nov. and C. tintin sp. nov. Together with C. dabagaensis Kraus, 1958, they constitute the Chaleponcus dabagaensis-group, well characterized by apparently apomorphic gonopodal characters, presumably monophyletic, and the first example of a major radiation within the Udzungwas. All species are restricted to altitudes >1390 m, all but one were found in only one, rarely two forest reserves, and the vast majority of specimens were collected in montane forest. Chaleponcus gracilior sp. nov. was collected in four forest reserves, often in secondary habitats where other species were only exceptionally found. Co-occurrence of multiple species, inter-specific differences in body size and unusual tarsal setation of a few species tentatively suggest adaptive radiation.