Six Nations Rugby announced on Thursday it had agreed to sell a 14 percent stake in the tournament to private equity giants CVC Capital Partners in a £365 million ($500 million) deal.
Tournament chiefs said the aim of the partnership was to grow the game and "attract a new, more diverse and global fan base".
The cash injection comes as the sport struggles with the devastating financial fallout from the coronavirus pandemic, with Six Nations matches this year taking place in empty stadiums.
The widely anticipated 425-million-euro deal with CVC, which already has significant investment in the English Premiership and Pro14, is subject to regulatory approvals.
"This is a hugely positive development and I want to express my thanks to all parties involved," Ben Morel, CEO of Six Nations Rugby, said in a statement.
"When we started this journey, our aim was to ensure we found the right strategic partner, who can add real value and is committed to a long-term relationship," he added.
"CVC recognises the exciting potential in the Six Nations championships and autumn international series, and they are aligned with our vision for the future."
The Six Nations statement said under the deal CVC would "invest up to £365 million for a one-seventh share in Six Nations Rugby, working alongside the rugby unions of England, France, Ireland, Italy, Scotland and Wales, which will together retain a six-sevenths share".
The funds will be paid to the six unions over five years, with the deal also covering the women's and under-20s' tournaments, plus the autumn international series.
CVC, which formerly owned a controlling stake in Formula One, is buying into rugby at a time when it desperately needs fresh funds.
The aim will be to boost revenues through improved commercial and broadcasting deals.
In the UK, the Six Nations is still shown on free-to-air television rather than on a paid-for service, giving one potential avenue for growth.
However, there are fears that going behind a pay wall could hamper ambitions to boost interest in the game as audience figures would fall.
Morel, however, told Britain's Press Association that the new investment was not a direct threat to terrestrial television coverage of the tournament.
"There is no correlation," he said. "CVC are interested in long-term value creation.
"Any decision over free-to-air, moving to pay... those decisions will remain in the power of the unions."
- 'Pivotal moment' -
Rugby chiefs welcomed the deal, which will be split on a sliding scale between the different unions, with the English Rugby Football Union (RFU) to receive around £95 million.
"We are delighted that the Six Nations unions have unanimously decided to partner with CVC," said Bill Sweeney, chief executive of the RFU.
"Their expertise will help to grow rugby union through the evolution of improved competitions and events and the growth of a more diverse fan base."
Welsh Rugby Union chief Steve Phillips described it as a "pivotal moment in the history of the international game in Wales".
"Ultimately, this deal will be a catalyst for the growth of our game," he said. "It will directly improve the international tournaments we participate in, further engaging new and existing fans alike."
Irish Rugby Football Union CEO Philip Browne said the announcement was "positive news" for the game in Ireland.
"Importantly, under the agreement, unions retain complete control of all sporting matters while all commercial and broadcast decisions, which will benefit from CVC's commercial and marketing expertise, also requires majority union approval," he said.