The bay-capped wren-spinetail (Spartonoica maluroides) is a species of bird in the Furnariinae subfamily of the ovenbird family Furnariidae.

Their upperparts are sandy brown to olive-brown with bold black streaks on the nape and upper back.

There are some minor differences in size and darkness of plumage, but little genetic divergence, between coastal and inland populations.

The species is monogamous, and both sexes build the nest, incubate the eggs, and brood and feed nestlings.

The nest is an open cup (uniquely among furnariids) built of twigs and grass and placed low to the ground deep inside marsh vegetation.

The bay-capped wren-spinetail's primary song is "a grasshopper-like buzz, long and tuneless, rising and falling away".

"This species is limited to wetland habitats, and is therefore highly threatened by marsh drainage and the drying effects of Eucalyptus and Pinus plantations, particularly in east Entre Ríos and north-east Corrientes [Argentina].