Which marine animal has the largest brain by mass?

The sperm whale has the largest brain by mass of any living animal.

Its brain can weigh about 8 kilograms, although exact values vary among individuals and measurement methods. That impressive mass does not mean the sperm whale is the most intelligent animal by every possible measure; brain size, body size, neural organization, and behavior are different topics.

Sperm whales are toothed whales with enormous heads dominated by the spermaceti organ. They use echolocation to locate prey, especially squid, in deep water. The species is also known for exceptionally deep and long dives.

The sperm whale should not be confused with the blue whale in questions about total body mass or length. The blue whale is the largest animal known to have lived, while the sperm whale holds the commonly cited living-animal record for brain mass.