Rugby's world body raised the residency period for Test eligibility from three to five years on Wednesday as it moved to stamp out controversial "player drain" from poor countries to richer rivals.
The amended regulation, which comes into effect at the end of 2020, comes after a review group decided rugby's current eligibility rules were "not in step with the modern game".
Rugby-mad Pacific island nations have been particularly hard-hit by the leaking of talent to the sport's richer, heavyweight countries, who frequently field naturalised players.
"This is an historic moment for the sport and a great step towards protecting the integrity, ethos and stature of international rugby," World Rugby vice-chairman Agustin Pichot said.
"National team representation is the reward for devoting your career, your rugby life, to your nation and these amendments will ensure that the international arena is full of players devoted to their nation, who got there on merit."
The new ruling takes effect after the next World Cup, in Japan in 2019, meaning it won't affect teams' current preparations.
New Zealand, England, Australia, Wales and France are among the top teams who regularly field players originally from Pacific countries such as Fiji, Tonga and Samoa.
Under the amended Regulation 8, which was approved at a special meeting of the World Rugby Council in Kyoto, players can also represent countries where they have cumulatively spent 10 years.
Anyone who plays sevens for a country aged 20 or over, or in the Olympics, becomes "captured" for that team and cannot represent another nation before fulfilling the five-year rule.
But there was no change to the "grandparent rule", where players are eligible to represent a country where one parent or grandparent was born.
The amendments follow a review of Regulation 8 "to consider whether it was in step with the modern professional rugby landscape", World Rugby said.
"The (review) group determined that Regulation 8 was not in step with the modern game, did not provide an adequate framework to protect the integrity of the international game and does not provide a deterrent to player drain from emerging rugby nations," it added.
World Rugby chairman Bill Beaumont called the reform "an important and necessary step to protecting the integrity and credibility of international rugby".