The boreal-arctic species Carex vaginata Tausch (Sheathed Sedge) has been found in Romania in 2014 during research on fen ecology. The species grows in the Rarau Mts., in an alkaline sloping fen of the Caricion davallianae alliance, transitional to Cratoneurion commutati springs (SE margin of Mt. Popii Rarăului, close to Plaiul Todirescu grasslands) and in a subalpine grassland at Mt. Popii Rarăului. Rarau Mts is the fourth locality for this species in the entire Carpathians, underlining its relictual distribution pattern in temperate Europe that may represent a remnant of formerly more continuous distribution during the glacial period. We further present a vegetation-plot record with Carex vaginata and basic chemical parameters of the water.