England will officially remain as sole hosts of the tournament but the Rugby Football Union had proposed using the iconic Cardiff venue due to its "strategic importance" as a 74,500-capacity stadium easily accessible from the rugby hotbeds in the English west country.
The IRB council endorsed that view today, confirming the Millennium Stadium will have a role at a third World Cup.
Wales were the main tournament hosts in 1999 and Cardiff staged pool matches and a quarter-final in 2007.
Paul Vaughan, chief executive of tournament organisers England Rugby 2015, said: "We are determined that Rugby World Cup 2015 will be the best tournament ever and to do that we want the best stadia available.
"Our commitment is to sell three million tickets at iconic and accessible venues. We are delighted that the IRB have listened to our case and that the Millennium Stadium can now be included in our planning for this global event."
The RFU plan to stage matches at many of England's leading stadia, including Wembley, the Emirates Stadium, Old Trafford and Anfield plus club rugby grounds Kingsholm and Welford Road.
Scotland's Murrayfield was not considered because of their record for poor attendances at previous World Cup matches.
IRB chairman Bernard Lapasset said: "The Millennium Stadium is one of the world's iconic sporting venues with a rich pedigree in hosting major sports events, including matches at Rugby World Cup 1999 and Rugby World Cup 2007.
"The venue is ideally positioned as a gateway to the tournament for fans from Wales and the south west and I am sure that it will add another exciting dimension for fans attending the 2015 Rugby World Cup."
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