Clamorous Reed Warbler is a large Song Thrush-sized warbler at 18-20 cm. The adult has an unstreaked brown back and whitish underparts. The forehead is flattened, and the bill is strong and pointed. The sexes are identical, as with most warblers. It is very like Great Reed Warbler, but that species has richer coloured underparts.

There are a number of races differing in plumage shades. The migratory northern race has the richest brown upperparts, and the endemic Sri Lanka subspecies is the darkest form.