''Octopus cyanea'', also known as the big blue octopus, day octopus and Cyane's octopus, is an octopus in the family Octopodidae. It occurs in both the Pacific and Indian Oceans, from Hawaii to the eastern coast of Africa.
''O. cyanea'' grows to 16 cm in mantle length with arms to at least 80 cm.
The type specimen was collected off Australia and is deposited at The Natural History Museum in London.