The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. The word abyss comes from the Greek word ἄβυσσος (ábussos), meaning "bottomless". At depths of 4,000–6,000 m (13,000–20,000 ft), this zone remains in perpetual darkness. It covers 83% of the total area of the ocean and 60% of Earth's surface. The abyssal zone has temperatures around 2–3 °C (36–37 °F) through the large majority of its mass. The water pressure can reach up to 76 MPa (750 atm; 11,000 psi).

As there is no light, photosynthesis cannot occur, and there are no plants producing molecular oxygen (O₂), which instead primarily comes from ice that had melted long ago from the polar regions. The water along the seafloor of this zone is largely devoid of molecular oxygen, resulting in a death trap for organisms unable to quickly return to the oxygen-enriched water above or to survive in the low-oxygen environment. This region also contains a much higher concentration of nutrient salts, like nitrogen, phosphorus, and silica, due to the large amount of dead organic material that drifts down from the ocean zones above and decomposes.

The region below the abyssal zone is the sparsely inhabited hadal zone. The region above is the bathyal zone.

The deep trenches or fissures that plunge down thousands of meters below the ocean floor (for example, the mid-oceanic trenches such as the Mariana Trench in the Pacific) are almost unexplored. Previously, only the bathyscaphe Trieste, the remote control submarine Kaikō and the Nereus have been able to descend to these depths. However, as of March 25, 2012 one vehicle, the Deepsea Challenger, had penetrated to a depth of 10,898 meters (35,756 ft).

The relative sparsity of primary producers means that the majority of organisms living in the abyssal zone depend on the marine snow that falls from oceanic layers above. The biomass of the abyssal zone actually increases near the seafloor as most of the decomposing material and decomposers rest on the seabed.

The composition of the abyssal plain depends on the depth of the sea floor. Above 4000 meters the seafloor usually consists of calcareous shells of foraminifera, zooplankton, and phytoplankton. At depths greater than 4000 meters shells dissolve, leaving behind a seafloor of brown clay and silica from dead zooplankton and phytoplankton. Chemosynthetic bacteria support large and diverse communities near hydrothermal vents, filling a similar role in these ecosystems as plants do in the sunlit regions above.

A new insight into the complexity of the abyssal environment has been provided by a team of researchers from the Scottish Society of Marine Sciences. They have found that manganese nodules on the deep sea floor produce free oxygen from water molecules.

The manganese nodules act as a kind of battery as they contain different metals, and they release oxygen into the environment. Because it was previously thought that only plants and algae produce dark oxygen (oxygen produced without light), this can be seen as a scientific breakthrough.