An endoparasitic platyhelminth from six species of sacoglossan opisthobranchs was collected at several localities of temperate to subtropical waters in Japan. Poecilostomatoid copepods (all species of Splanchnotrophidae and several species of Philoblennidae) and a few digenean flukes had been the only endoparasitic metazoans known for opisthobranch hosts. The newly discovered parasite was 1 to 15 mm in length and had no eyes, mouth, pharynx, or intestine. It had no external organs for parasitic life (e.g. attachment organs) and inhabited the haemocoel of the host. When mature, it emerged from the host and secreted a silky substance around itself to form a cocoon. The cocoon contained egg capsules with 19–42 eggs. Larvae, hatched from the capsule, had a ciliated body and a pair of eye spots. They were negatively phototactic and capable of invading suitable hosts. These morphological and life history features suggest this parasitic worm may belong to the family Fecampiidae (Platyhelminthes: Turbellaria), one of a few obligate parasite taxa in Turbellaria. Molluscan hosts which are common for parasitic Platyhelminthes have not previously been known for this family. The newly discovered parasite may be important for understanding the evolution of parasitism in Platyhelminthes.