On the basis of bibliographical, morphological and molecular studies (SSU rDNA), the reinstatement of species rank for Chaetomorpha stricta Schiffner, a poorly known Mediterranean endemic species described in the early 20th century, is proposed. Previously reduced to a posterior heterotypic synonym of C. linum (O.F. Müller) Kützing, C. stricta clearly differs from the latter by its small, light green, free-living filaments entangled in dense masses, the cells 0.5 to 1.6 (rarely 2) times as long as broad, with cell walls lamellate and broad up to 75–90 μm thick, and by molecular data. Chaetomorpha stricta was rediscovered, in May 2011, in oligotrophic clear waters of a closed fishing pond in the Valle Cavallino (northern basin of the Venice Lagoon). A critical review of Chaetomorpha taxa reported in the Mediterranean Sea and a taxonomic key to the Mediterranean taxa currently accepted are presented.