A large chaetopterid polychaete, Mesochaetopterus rogeri sp. nov. is described as new from the Mediterranean Sea. The analyses of partial sequences from the nuclear 18S rRNA (643bp) and the mitochondrial Cytochrome Oxidase I (577bp) genes of representative individuals of all known chaetopterid genera indicated the initial assignment of the new species into Mesochaetopterus. These analyses also supported the monophyly of the family and revealed two well-supported clades: Chaetopterus / Mesochaetopterus and Spiochaetopterus / ,Phyllochaetopterus. Mesochaetopterus rogeri sp. nov. was close to M. xerecus, here re-described from newly collected material. Mesochaetopterus rogeri sp. nov. was characterized by: 1) two long tentacles with dorsal transversal black bands with alternating widths (sometimes with two additional longitudinal light-brown bands); 2) A region with nine chaetigers (up to 12), with 13 - 19 modified chaetae in the 4th; 3) B region with three flat segments, with accessory feeding organs in the 2nd and 3rd; 4) sandy straight tubes, 2.5 m long or more, vertically embedded in the sand. In the Bay of Blanes, M. rogeri sp. nov. occurs between 6 and 9 (up to 30) m deep, with a patchy distribution (< 1 ind. m-2), maximum densities in April/June (likely due to recruitment events) and minimum in September/November (likely a behavioural response to increasing sediment dynamics). Although it was originally thought that M. rogeri sp. nov. could be an introduced species, we argue that it is probably a native of the Mediterranean, which has been overlooked by scientists up to now.Peer reviewed