The black ruby barb (Pethia nigrofasciata, formerly Puntius nigrofasciatus) or purplehead barb is a tropical cyprinid fish endemic to Sri Lanka, where it occurs in forested streams from the Kelani basin to the Nilwala basin.

The brightly colored population introduced to Mahaweli at Ginigathena, Sri Lanka, is said to have diminished in number due to the aquarium export trade.

The silver body turns into a dark ruby-red color in mature, breeding males.

The fish will grow to a maximum length of 2–3 inches (5.1–7.6 cm) with the female being slightly longer than the male.

Black ruby barbs natively live in a tropical climate among cool, shady, quietly flowing forested streams with either gravel or sand substrates.