The Millerbird (Acrocephalus familiaris) is a species of Old World warbler in the family Acrocephalidae. It had two subspecies, A. f. kingi and A f. familiaris. The latter, the Laysan Millerbird, became extinct sometime between 1916 and 1923. The former, the critically endangered Nihoa Millerbird, remains the only race left, inhabiting the small island Nihoa in Hawaii. It is the only Old World warbler to have colonised Hawaii, although there is no fossil evidence that the species ever had a distribution beyond these two islands.
Millerbirds form long-term pair bonds and defend territories over a number of years. Territories can be as large as 0.95 ha although 0.19–0.40 ha is more typical. Breeding occurs variably from January to September depending on food availability.