As introduced species constitute a major threat to biodiversity, it is crucial to properly monitor their spread to new regions. The present study reports new records of four species: 1) the amphipod Ampithoe valida S.I. Smith, 1873 and 2) the polychaete Polydora colonia Moore, 1907, both new records for Brittany (North-West part of France); 3) the polychaete Bispira polyomma Giangrande & Faasse, 2012, a first finding in the North-East Atlantic; and 4) the flatworm Prostheceraeus moseleyi Lang, 1884 with a confirmed northernmost record in North-East Atlantic. These species were recorded within two semi-enclosed bays: the Morbihan Gulf on the southern coast of Brittany and the Bay of Brest in the North-West part of Brittany. Both localities are already known to host numerous non-indigenous species as well as benthic macrofaunal distributional range limits. Morphological variations and inconsistencies are discussed and reported for A. valida, revealing missing characters and errors in the literature. This work adds three non-indigenous species to Brittany and two nonindigenous species to French waters, for which we discuss potential introduction vectors.