Paranotothenia magellanica was first formally described as Gadus magellanicus in 1801 by the German naturalist Johann Reinhold Forster with the type locality given as the Straits of Magellan.

[4] Paranotothenis magellanica has a wide head with a short snout and a mouth which reaches as far back as the middle of the eye.

The upper body is dark blue, greyish-green, brown or black while the abdomen is cream, gold-yellow or reddish.

Adults elsewhere appear to stay close to shore and may also be pelagic in habit to some extent as they are rarely captured in large bottom trawls.

Off the Kerguelen Islands spawning appears to occur in April and May, a female may lay 60,000-70,000 pelagic eggs which are about 0.8 mm in diameter.

Magellanic rockcod feed have been recorded to eat the algae, amphiipos, the crab Halicarcinus planatus, the squat lobster Munida gregaria, copepods, isopods, hydrozoans, larval crustaceans and molluscs, including the bivalve Gaimardia trapesina.