Eugenia koolauensis, commonly known as Koʻolau eugenia or nioi, is a species of flowering plant in the myrtle family, Myrtaceae. It is endemic to Hawaii, where it could previously be found on the islands of Molokaʻi and Oʻahu; today populations only exist on the latter. This is a federally listed endangered species of the United States.

It is one of two Eugenia species native to Hawaii, and the only endemic. It is a shrub or tree grows 2 to 7 meters tall. The tips of the branches and the undersides of the leaves are hairy. White flowers occur in the leaf axils

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