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.