The Saipan reed warbler or gå'ga' karisu in Chamorro (Acrocephalus hiwae) is a critically endangered songbird of the Northern Mariana Islands.

It is sometimes considered a subspecies of the extinct nightingale reed warbler (A. luscinius) by some taxonomists.

The Saipan reed warbler is approximately 17 cm (6.7 in) long, and is greyish olive-brown above with a pale-yellow underside.

Both sexes have a long bill compared to other reed warbler species.

Threats to the survival of the Saipan reed warbler include habitat destruction resulting from urban development and agriculture, the introduction of invasive species, and volcanic eruptions.