The Stadium's green credentials already include collecting rainwater to water the pitch, recycling throughout the Stadium, a "Cycle to Work Scheme" for employees and are currently working along side energy companies to compile an 'Energy Saving Guide' for member rugby clubs to advise ways they can save money by saving energy.
Millennium Stadium Manager Gerry Toms said: "The Millennium Stadium is very proud to be taking part in WWF's Earth Hour and to be joining such a prestigious list of landmarks and buildings in doing so.
"We take environmental issues very seriously at the Stadium and are pleased to be adding to our green credentials by signing up to WWF's Earth Hour initiative. We would like to urge all our member rugby clubs, fans and the people of Wales to take part in switching their lights off for just one hour in a demonstration for action on climate change."
WWF Cymru is delighted that one of the UK's most recognised sporting and music venue will be supporting Earth Hour.
Morgan Parry, Head of WWF Cymru, said: "We are thrilled to have the Millennium Stadium on board and signed up to Earth Hour. Having such a distinguished and iconic landmark which lies in the heart of Cardiff committed to Earth Hour will most certainly make Wales a leading country in Earth Hour 2009 and help make it a successful global event.
"By being involved in this international campaign the Stadium will help WWF Cymru demonstrate to leaders in Wales and across the world that people want them to tackle climate change."
For further information on Earth Hour and how to get involved visit www.wwf.org.uk/earthhourwales.