Alternanthera nahui, common name nahui, is a species in the family Amaranthaceae, native to New Zealand and to Norfolk Island.
It is a perennial herb that has a slender tap root. Its stems are 1.5–3.0 mm in diameter and tend to lie down. It can be distinguished from Alternanthera sessilis by its narrower leaves, its keeled tepals, its shorter staminodes and style.
It was first described in 2009 by Peter Heenan, Peter de Lange and J. Keeling.
There are no synonyms according to Plants of the World Online. However, according to NZPCN it has, from time to time, been incorrectly referred as Alternanthera sessilis (L.) Roem. & Schult.
It is a coastal and lowland species found in both seasonally and permanently wet habitats.
In the 2018 conservation assessment of de Lange and others under the New Zealand Threat Classification System it was classed as "Not Threatened", having a "large, stable population".[1]