The thick-billed warbler (Arundinax aedon) breeds in the temperate east Palearctic, from south Siberia to west Mongolia.

This passerine bird is a species found in dense vegetation such as reeds, bushes and thick undergrowth.

The adult has an unstreaked brown back and buff underparts, with few obvious distinctive plumage features.

The song is fast and loud, and similar to the marsh warbler, with much mimicry and typically acrocephaline whistles added.

It was sometimes placed in the monotypic genus Phragmaticola (or Phragamaticola) and for a long time as Acrocephalus and in 2009 suggested as being within the Iduna clade.