The River Warbler, Locustella fluviatilis, is an Old World warbler in the grass warbler genus Locustella. It breeds in east and central Europe into western Asia. It is migratory, wintering in east Africa.

This small passerine bird is a species found in dense deciduous vegetation close to water in bogs or near a river. 5-7 eggs are laid in a nest in a tussock or on the ground. This species is a rare vagrant to western Europe.

This is a largish warbler. The adult has an unstreaked grey-brown back, whitish grey underparts, and a darker undertail, which has white feather tips giving a contrasting pattern. The sexes are identical, as with most warblers, but young birds are yellower below. Like most warblers, it is insectivorous.

This is a skulky species which is very difficult to see except sometimes when singing. It creeps through grass and low foliage.

The song is a monotonous mechanical insect-like reeling, often given at dusk. It is similar to the song of a Grasshopper Warbler, but has more of a sewing machine quality, and may be produced for long periods.