Which marine mammal can hold its breath the longest?

The marine mammal that can hold its breath the longest is Cuvier's beaked whale.

A tagged Cuvier's beaked whale completed a recorded dive lasting 222 minutes—3 hours and 42 minutes—setting the mammalian record for the longest known breath-hold dive. The species also reaches extraordinary depths, with recorded dives approaching 3,000 meters.

These whales survive such dives through powerful adaptations: unusually high concentrations of oxygen-storing myoglobin in their muscles, slowed heart rates, reduced blood flow to nonessential tissues, and collapsible lungs that help withstand immense pressure. They hunt deep-water squid and fish using echolocation.

Sperm whales are famous deep divers and can remain underwater for well over an hour, but their recorded maximum is shorter. Blue whales are larger overall, yet they generally make shallower, shorter dives. The record applies to a documented dive by a wild animal; it is not a claim that every Cuvier's beaked whale routinely stays submerged this long.