This season's Guinness Pro14 is set to conclude in March with the final played between the winners of Conference A and Conference B.
These placings will also be used to decide qualification for next season's European competitions.
A new tournament, dubbed the "PRO14 Rainbow Cup", involving 16 teams in total - four of which will be from South Africa - will then run from 17 April until 19 June, when its final will take place. The Vodacom Bulls; Emirates Lions; Cell C Sharks and DHL Stormers are the non-European teams that will take part.
The format will be similar to the Pro14 - two groups of eight, and a final between the top team in each, with two Irish, two South African, two Welsh, one Italian and one Scottish club in each conference.
The competition will also provide matches for the South African teams in advance of the British and Irish Lions' tour of the country in the summer.
"At a time where our sport has faced its greatest challenge, we have promising news about an innovative solution to partner with South African rugby ahead of the British & Irish Lions tour," Martin Anayi, CEO, PRO14 Rugby said.
"It was vital that we worked with all parties to ensure our 12 teams had a clear road towards European qualification. With a Lions tour in South Africa to come it is hard to think of anything better to whet the appetite than the best players from the Celtic regions competing against World Cup-winning Springboks in the Guinness PRO14 Rainbow Cup."
Jurie Roux, CEO, SA Rugby added: "The inclusion of South Africa’s ‘Super’ Teams in the Rainbow Cup is a once-in-a-generation, watershed moment for South African rugby. After so much turmoil and uncertainty in 2020, the prospect of a return to top-flight international domestic competition is one to which we all can all look forward with real excitement."
SA rugby voted in September to allow four of its leading franchises to compete in an expanded version of the old Celtic League.
Listen to the RTÉ Rugby podcast on Apple Podcasts, Soundcloud, Spotify or wherever you get your podcasts.
Follow Munster v Leinster (Saturday 7.35pm) and Connacht v Ulster (Sunday 7.35pm) via our live blog on rte.ie/sport or listen on RTÉ Radio 1.
