Marionfyfea adventor is a species of land planarian described in 2016 from specimens found in the United Kingdom, the Netherlands and France.

The dorsal and lateral surfaces are irregularly patchy light and dark brown colour with small iridescent blue spots.

The creeping sole is uniform paler brown and about 40% of body width.

Marionfyfea adventor has been recorded in widely scattered localities in the United Kingdom, Netherlands, France, Germany, Denmark and Ireland.

Its widely scattered distribution suggests that it may have been in Europe for some considerable time.