Which constellation is named after a southern triangle?

The constellation named after a southern triangle is Triangulum Australe.

Its name is Latin for “southern triangle,” distinguishing it from Triangulum, the Northern Triangle. The three brightest stars—Alpha, Beta, and Gamma Trianguli Australis—form a compact, roughly equilateral pattern in the far southern sky. Alpha Trianguli Australis is also called Atria.

The constellation first appeared on a celestial globe made by Petrus Plancius in 1589 under the name Triangulus Antarcticus. Johann Bayer gave it a more accurate depiction and its present Latin name in his 1603 star atlas, Uranometria. Nicolas-Louis de Lacaille later charted its brighter stars in 1756.

Triangulum Australe is often confused with Triangulum because both names refer to triangular star patterns. The “Australe” qualifier is the key: Triangulum lies in the northern celestial hemisphere, while Triangulum Australe is a southern constellation. It is bordered by Norma, Circinus, Apus, and Ara, and appears on the flag of Brazil as one of the represented constellations.