The Iberian Peninsula is part of the South European Atlantic Shelf within the Lusitanian ecoregion. Given the characteristics of this region, a great invertebrate biodiversity is expected. Nevertheless, no literature records of Polycladida are known for the Cantabrian Sea. Here, we report the presence of six polyclad species, including one new species. Notoplana vitrea, considered endemic to the Mediterranean Sea, was found in the Cantabrian Sea, demonstrating its presence in Atlantic waters. This species was previously reported for these waters on two natural history photographic websites: the importance of searching, indexing and disseminating this type of record for the scientific community is discussed. Discocelis tigrina is reported for the first time for the Cantabrian Sea, and is the northernmost record to date. In this paper, Pleioplana atomata is reported for the second time for the Iberian Peninsula, yet is the first record for the Cantabrian Sea. Although a literature record of Leptoplana tremellaris for the Iberian Peninsula exists, it is considered a misidentification of L. mediterranea; therefore, this work provides the first record of L. tremellaris for the Iberian Peninsula. The cosmopolitan species Cycloporus papillosus is also reported for the Cantabrian Sea. A new species, Imogine fafai sp. nov., is described and taxonomically compared with other species of the genus.