Subnautica: Below Zero is an open-world survival action-adventure video game developed and published by Unknown Worlds Entertainment. The game is a spin-off to Subnautica.

Introduced in early access via Steam and the Epic Games Store in January 2019, Subnautica: Below Zero was released for macOS, Nintendo Switch, PlayStation 4, PlayStation 5, Windows, Xbox One, and Xbox Series X/S on May 14, 2021. The physical versions of the game were published by Bandai Namco Entertainment. It received generally positive reviews from critics.

Subnautica: Below Zero is a survival adventure game set in an open world environment and played from a first-person perspective. The player controls xenologist Robin Ayou, who lands on ocean planet 4546B in pursuit of answers about her sister's mysterious death. Like its predecessor, gameplay involves exploring and surviving in alien environments while also completing objectives to advance the game's plot. Players collect resources, construct tools, build bases and submersibles, and interact with the planet's wildlife.

To survive and progress, the player must collect raw materials from the environment, including various minerals, flora samples, and fish, while avoiding or countering threats like hostile fauna and blinding snowstorms. They must also acquire blueprints to craft new items, either through finding and opening databoxes in the world or by scanning fragments of the item. Generally, the player must face harsher environments the further they progress, requiring better equipment or upgrades for old ones, like suits that resist the cold, oxygen tanks with higher capacities, or vehicles that can withstand deeper waters.

The game primarily takes place underwater, but, unlike its predecessor, there are much more expansive and dangerous land-based areas to explore. There are structures and wrecks to find both above ground and underwater, which serve as key locations for the story, providing blueprints and recordings of conversations that slowly reveal lore.

Alongside returning mechanics such as oxygen, hunger, and thirst, players now have a body heat gauge which comes into play when walking on land. It functions much like the oxygen gauge does underwater. The player will gradually lose body heat and freeze unless they replenish their body heat by jumping into water, finding shelter, consuming certain items or standing next to a heat source.

Upon beginning a new game, players must select a difficulty mode from the following five:

Two years after the events of the original game, the Alterra corporation that originally crashed the Aurora on planet 4546B had built several bases on the planet for research on the Kharaa bacterium, and on the alien precursor race, dubbed Architects, that had studied the bacterium prior. Xenologist Robin Ayou smuggles herself onto the planet in a frozen sector named "Sector Zero" to investigate the circumstances of her sister Sam's death, which Alterra claims was a result of "employee negligence." During the past year, Alterra had recently withdrawn all of its personnel, allowing Robin to land on the planet.