This study reports four alien polychaete species new to the marine fauna of Greece, four of which are aliens. These species are -Pseudopolydora paucibranchiata (family Spionidae), Paraprionospio coora (family Spionidae), Marphysa disjuncta (family Eunicidae), and Chaetozone corona (family Cirratulidae). Another species of Chaetozone, though not an alien one, Chaetozone gibber is reported here from Greek waters. All of them have been currently reported from the coasts of Turkey (Aegean or Levantine Seas). Pseudopolydora paucibranchiata and Marphysa disjuncta, probably introduced to the region through ballast waters, seem to be well established in the Mediterranean Sea. Chaetozone corona and Paraprionospio coora were considered to be cryptogenic. Pseudolydora paucibranchiata was found in great densities in a very disturbed site, confirming the opportunistic character of this species. Chaetozone corona and Chaetozone gibber showed a wider distribution pattern, though their abundance increased in disturbed sites. Marphysa disjuncta was found in disturbed as well as in undisturbed sites along the coasts of Greece and Paraprionospio coora is characteristic of moderate to higher depth zones. These new findings increase the number of polychaete species from the Hellenic Seas to 777 species and the number of alien species to 33.