The Marsh Antwren (Stymphalornis acutirostris), also known as the Paraná Antwren, is a species of bird in the Thamnophilidae family. It is monotypic within the genus Stymphalornis, but possibly better included in Formicivora. It is endemic to marshes and swamps in the Brazilian states of Paraná and Santa Catarina. A closely related and rather similar taxon has recently been discovered near São Paulo, it already being known by the common name of São Paulo Antwren. A scientific description is currently being prepared.
It is threatened by habitat loss and currently considered Endangered by BirdLife International.