Maritrema madrynensis n. sp. is described from the Patagonian coast, Argentina, based on naturally obtained adults from the kelp gull Larus dominicanus and cultured metacercariae from the crab Cyrtograpsus altimanus. The new species fits into the “eroliae complex” and can be distinguished from other related species mainly in shape, size, and distribution of the spines on the cirrus and vitellarium in a complete ring in all specimens. Based on morphological and morphometric features of the naturally and experimentally obtained specimens and the trophic relationship between invertebrate and vertebrate hosts, we suggest that the crab C. altimanus acts as a second intermediate host of M. madrynensis n. sp. This is the first report of a Maritrema species in Patagonia and the southernmost record for the genus.