The Pro14 has agreed a deal to sell a 28% stake in the league to private equity firm CVC Capital Partners.
CVC will buy a minority shareholding after agreeing a similar deal with English clubs.
The deal will bring major investment, believed to be about £120m, that will benefit the Irish, Scottish, Welsh and Italian Rugby Unions that form Celtic Rugby DAC, which runs the league.
The Unions will retain the 72% majority share of the tournament.
The investment will be distributed as an equal share across all four partners with each union receiving £30m each.
Pro14 bosses have been in long-term discussions with CVC after the firm bought a stake in the Premiership in December 2018, a deal worth more than £200m for a 27% stake.
Under this agreement, CVC Fund VII will acquire the 28% minority share and Pro14 chief executive Marin Anayi will continue in his role.
The Pro14 covers five countries and 14 clubs across Wales, Scotland, Ireland, Italy and South Africa.
Celtic Rugby DAC - responsible for running and operating the league - is owned by the Irish Rugby Football Union (IRFU), the Scottish Rugby Union (SRU) and Welsh Rugby Union (WRU) and now the Federazione Italiana Rugby (FIR), while members of the South African Rugby Union are on the board.
As part of this agreement, the FIR has become a member of Celtic Rugby DAC and will receive a share of the investment, while some funding will also be held centrally by Pro14 Rugby.
CVC have extensive experience of prior funds investing in multiple sports businesses, such as Formula 1, Moto GP and Premiership Rugby.
"CVC's show of faith has been impressive and is in keeping with their proven track record of success when it comes to sports investment," said Anayi.
"This partnership allows all of our stakeholders to plan for a sustainable period of growth, which will benefit the fans, the players and the game."
Dominic McKay, chairman of Celtic Rugby DAC and chief operating officer of Scottish Rugby, welcomed the move.
"As a board we have been ambitious in our outlook and have significantly developed the league in recent years," said McKay.
"Sport, like all of society is dealing with major challenges currently that we could not have imagined just a few months ago."
Welsh Rugby Union chief executive Martyn Phillips added: "It is to the credit of everyone involved the deal is now over the line and we look forward to a sea change in the ability of the Pro14 to realise its full potential.
"This investment is great news for our teams and for Welsh rugby as a whole, although we are under no illusions that Covid has and will continue to have a significant impact on our organisation for some time."