The organisation's general assembly announced the French territory's admission in Paris in the presence of New Caledonia's caretaker president Philippe Germain.
The French overseas minister Annick Girardin said it allowed New Caledonia to deepen its work in the region, nine years after its marine areas became a world heritage site.
New Caledonia was made an associate member only because it is not in control of its foreign affairs.
As an associate member, New Caledonia has no voting rights.
In a year, the territory is due for an independence referendum.