In North America, where it is known as the Boreal Owl, it was named after the Greek god of the north wind, Boreas. In other parts of the world, it is known as Tengmalm's Owl. Colouration varies widely between individuals, with some having reddish-brown hues, while others are more greyish. The facial disc is whitish, surrounded by a dark rim with small white spots. There is a small dark portion between the eyes and the base of the bill.
Eyes are pale to bright yellow, and the bill is yellowish horn.
Upper parts are dark brown with bold white spotting. Underparts are off-white, with broad streaks of darkish brown, denser on the breast and trailing off at the lower belly. The legs and feet are covered with white feathers. Claws are darkish horn to blackish brown, and have very sharp black tips. This Owl is mainly nocturnal.