The western olivaceous warbler, also known as isabelline warbler, (Iduna opaca ) is a "warbler", formerly placed in the Old World warblers when these were a paraphyletic wastebin taxon. It is now considered a member of the acrocephaline warblers, Acrocephalidae, in the tree warbler genus Iduna. It was formerly regarded as part of a wider "olivaceous warbler" species, but as a result of modern taxonomic developments, this species is now usually considered distinct from the eastern olivaceous warbler, Iduna pallida.

It is a small passerine bird, found in dry open country, including cultivation, with bushes or some trees. 2-3 eggs are laid in a nest in low in undergrowth or a bush. Like most warblers, western olivaceous warbler is insectivorous.

It is a medium-sized warbler, more like a very pale reed warbler than its relative the melodious warbler. The adults have a plain pale brown back and whitish underparts. The bill is strong and pointed and the legs grey. The sexes are identical, as with most warblers, but young birds are more buff on the belly. It has a characteristic downward tail flick.

Western olivaceous warbler breeds in Iberia and north Africa. It is migratory, wintering in sub-Saharan Africa. It is a rare vagrant to northern Europe.

Western olivaceous warbler is larger and has a browner tinge to the upperparts than eastern olivaceous warbler. It also has a larger bill. The song is a fast nasal babbling.