TV3 has secured the rights to the Six Nations Championship for a period of four years from 2018 until 2021.
CEO of Six Nations Rugby John Feehan said: "We are delighted to be working with TV3 on developing the interest in the Championship and to ultimately grow this wonderful competition in Ireland.
"TV3's coverage of Rugby World Cup was very successful and we believe that they will inject new and exciting ideas in the broadcasting of the Six Nations Championship.
"I wish to acknowledge the excellent work and support of RTE in the coverage of the RBS 6 Nations Championship over the years, and indeed we look forward to working with them for the remaining two years of the existing contract."
The Six Nations will be broadcast on RTÉ for the next two years under the current rights agreement.
In a statement, RTÉ Group Head of Sport Ryle Nugent said: "This is extremely disappointing news for RTÉ.
"We have a long and proud tradition of showcasing the Six Nations and putting our heart and soul into our coverage.
"We want to assure the Irish public that every possible effort was made by RTÉ to retain these rights. As a public service broadcaster, RTÉ’s ambition has always been, and continues to be, to deliver the best possible viewer experience and to share moments of major national importance.
VIDEO: RTÉ Group Head of Sport Ryle Nugent responds to TV3 winning the rights to the Six Nations from 2018. https://t.co/8u5nl0zXDR— RTÉ Rugby (@RTErugby) November 11, 2015
"We put forward the best possible bid within our means, while always mindful of the significant responsibilities attached to spending public funds.
"We have a phenomenal year ahead in 2016, with the GAA Championships, the Olympic Games and Euro 2016. We also look forward to delivering the Six Nations Championship in 2016 and 2017.
"Along with the range of sports we have already secured and are delivering, we will look to other opportunities in Irish and international sports and remain committed to delivering a comprehensive portfolio of sport to the Irish public."