The genus Mediomastus is redefined, and nine species are recognised. Three of these are new species, and one, M. deductus, is transferred from the genus Heteromastus. Three further species probably belong in Mediomastus but there is insufficient information available to confirm this. Two further species originally placed in Mediomastus are shown to belong in other genera. A full description of each species is given, based upon the re-examination of type material. Two tables indicating all the relevant diagnostic characters of all described species of Mediomastus are given. Comments are made on the validity of the characters that have been used traditionally in capitellid taxonomy. Setal characteristics are the most diagnostic and these should be examined by SEM as well as by light microscopy. However, identification of species of Mediomastus requires a combination of diagnostic characters.