The marsh antwren (Stymphalornis acutirostris), also known as the Paraná antwren, is a insectivorous bird in the antbird family Thamnophilidae. It is endemic to marshes and swamps in the Brazilian states of Paraná and Santa Catarina.
It was first described in 1995. Although initially placed in its own genus Stymphalornis, a subsequently molecular phylogenetic study has shown that the marsh antwren is closely related to species in the genus Formicivora.
The subspecies S. a. paludicola was discovered near São Paulo and described in 2013 as a new species in the genus Formicivora. This subspecies has been found in fifteen small, isolated areas in the east of the state of São Paulo, not far from the city of São Paulo, in south-eastern Brazil. The sites lie within the headwaters of the Paraíba do Sul and Tietê Rivers, at elevations of 600–760 m (1,970–2,490 ft). There it inhabits marshes with relatively tall (60–250 cm (24–98 in)) vegetation. Much of its habitat is degraded and threatened by agricultural, industrial and residential development.
Males of the subspecies S. a. paludicola can be distinguished from those of the nominate subspecies by their black underparts and thighs, very dark grey-brown upperparts, and a smaller exposed culmen. Females are distinguished by their very dark grey-brown upperparts and flanks, and smaller exposed culmen.
The fourth edition of the Howard and Moore Complete Checklist of the Birds of the World and the online edition of the Handbook of the Birds of the World both treat the two subspecies as separate species and place them in the genus Formicivora. In their treatment Formicivora acutirostris is given the English name "Parana antwren" and Formicivora paludicola the name "marsh antwren".