A new species of digenetic trematode, Apocreadium galaicus (Apocreadiidae), parasitic in the intestine and occasionally the stomach of the thickback sole Microchirus variegatus (Soleidae, Osteichthyes), is described. To date, Apocreadium has been reported only from American and Asian coastal waters; this is the first report of a species from European coastal waters. The most similar taxa to the new species are A. foliatum, A. mexicanum, A. synagris and A. balistis. A. galaicus can be distinguished a) from A. foliatum, because the oral sucker of this species is larger than the acetabulum, while in A. galaicus it is smaller; b) from A. mexicanum by the simple (not sphincter-like) structure of the anterior quarter of its pharynx and by its larger eggs, c) from A. synagris by the position of the oral sucker and the restriction of cuticular spines to the pretesticular region in A. galaicus and d) from A. balistis by its longer post-testicular region (55.3 - 69.6% of body length), by the densely spiny cuticle of the acetabular region and its larger eggs. In the study area, prevalence of infection by A. galaicus showed a clear peak between November and January, and dropped to zero in May / June.