Macrostomum rostratum is a free-living, hermaphroditic flatworm in the family Macrostomidae, found in freshwater and brackish environments.

There are a pair of small eyes at the anterior end and a longitudinal-slit mouth on the ventral surface just behind these.

[2][3] Macrostomum rostratum occurs sporadically in the European coastline in the northeastern Atlantic Ocean, typically in the splash zone in both fresh and salt water.

It is present in rock pools at Oxwich Bay and Port Erin in Wales, and in these locations it is found on sea lettuce (Ulva lactuca).

[6] The structure of the male copulatory device with its curved stylet may indicate that this species exhibits a hypodermic mating system.