The International Rugby League has elevated Tonga and Samoa to Tier 1 status, alongside Australia, New Zealand and England.
The IRL also approved a major eligibility change, allowing players to represent any nation where a great-grandparent was born.
The rule applies to all three forms of the game and every nation, IRL chair Troy Grant said.
The decision followed a "detailed submission from the Cook Islands," the IRL said in a Monday morning press release.
Grant said the board discussed the application at length and reviewed analysis and data before reaching its decision.
The Daily Telegraph's Dave Riccio reported the change on Channel 7's World Cup coverage on Sunday night.
Riccio said the decision would "future-proof" the international game.