Ulva pertusa Kjellman (Ulvales, Chlorophyta), which is widely distributed in the Pacific Ocean, has been detected on the NW Atlantic coast of the Iberian Peninsula (Galicia, Spain). The material has lobed and perforated blades, without marginal teeth, and with characteristic wrinkling at the base; it has an average thickness of 150 μm in the upper basal zone. Cells have a single chloroplast with (1-) 2 (-3) pyrenoids and, in surface view, show only local ordering. In transverse sections, cells are squarish or oblong, with rounded corners. Blades have a wide fertile margin with cells containing up to 28 gametes. The species-level identification as U. pertusa was confirmed by molecular studies (rbcL sequence analysis). This is the first report for this opportunistic species from the Iberian Peninsula. Its introduction is probably attributable to the importation of biological material for shellfish culture.