Ocean colonization (also blue colonization or ocean grabbing) is the exploitation, settlement or territorial claim of the ocean and the oceanic crust.

Ocean colonization has been identified critically as a form of colonization and colonialism, particularly in the light of growing exploitive and destructive blue economy ocean development, such as deep sea mining, calls for blue justice have been made.

Ocean colonization as ocean settlement, or seasteading, being the extending of human settlement to the ocean, has been identified as settler colonial "tech-colonialism" at sea. Such settlements have been suggested to be established with floating accommodation platforms, such as very large cruise ships or artificial islands, establishing seasteads, or with underwater habitats, employing offshore construction, with arguments for floating structures, as they are generally less impacted by natural disasters. Ocean settlement with the construction of artificial structures in aquatic environments though can also be disruptive to natural marine ecosystems.

Territorial claims are another and continuing international issue, with sovereign states advancing claims through developing and claiming uncontrolled islands, such as in the South China Sea and ocean settlements possibly establishing sovereign states.

Ocean colonization has been advocated for and compared to space colonization, particularly as a proving ground for the latter. In particular, the issue of sovereignty may bear many similarities between ocean and space colonization; adjustments to social life in harsh circumstances would apply similarly to the ocean and to space; and many technologies may have uses in both environments.

The law of the sea internationally negotiated in the latter half of the 20th century states that the ocean is the "common heritage of humanity". From this the need for an international regulation regime has been identified and negotiated for. As the ocean was seen as a technologically optimistic futurist reservoir for economic growth, a model of internationally shared exploitation through a body called the "Enterprise" clashed with private commercial exploitation perspectives.

In addition to this economic dimension, environmental protection considerations have produced calls for rights of nature for the ocean.

Underwater habitats are examples of underwater structures.