The Great Reed Warbler, Acrocephalus arundinaceus, is an Old World warbler in the genus Acrocephalus. It breeds in Europe and westernmost temperate Asia. It is migratory, wintering in tropical Africa. It does not breed in Great Britain, but is a regular visitor.

This passerine bird is a species found in large reed beds, often with some bushes. 3-6 eggs are laid in a basket nest in reeds.

This is a large Song Thrush-sized warbler, 16-20cm in length. The adult has an unstreaked brown back and buff underparts. The forehead is flattened, and the bill is strong and pointed. It is very like a giant Reed Warbler, but with a stronger supercilium. The sexes are identical, as with most warblers, but young birds are richer buff below. Like most warblers, it is insectivorous, but will take other small prey items.

The song is very loud and far carrying. It is a slow, chattering jit-jit-jit with typically acrocephaline whistles and mimicry added.