A new species of Lejeuneaceae, Harpalejeunea zilmarii, from an upper montane forest in southeastern Brazil, is described and illustrated. The new species is characterized by the upper leaf which has an ovate, asymmetric lobe, is narrowed toward the apex, with an acuminate apex, a uniseriate tip that is 2–4 cells long, an entire margin, or with cells of the proximal part of the antical margin (up to ca. 2/3 lobe length) crenate-denticulate by conical projections, constricted lobules forming a short and slightly curved passage to the water sac, with 2 short, blunt, closely associated single-celled teeth, and intercalary androecia on the main stem. The new species resembles Harpalejeunea tridens, and its affinities are discussed and presented in the key to the Brazilian species of Harpalejeunea.