During a survey of the species of Poaceae in Serra do Ouro Branco in the southern Espinhaço Range, State of Minas Gerais, Brazil, a new species of Isachne (I. hirtiglumis) was found and is described here. This new species differs from the other three species of the genus occurring in Brazil mainly by the hirsute indument of the glumes. Isachne hirtiglumis grows in swamps and wet areas associated with gallery forests within the campos rupestres. Besides the description and illustrations, an identification key for the species of Isachne, confirmed for Brazil to date is provided and nomenclatural problems are discussed.