The game will be played on Monday May 23, 2005. Kick off time has yet to be confirmed.
It will be the seventh time that the touring side have faced Argentina but the first encounter within the British & Irish Isles.
Commenting on the confirmation of the fixture British & Irish Lions Chairman and Tour Manager, Bill Beaumont, said: "It is a key match for the Lions as Argentina will provide a tough pre-tour fixture and will set the tone for the challenges that will face us in New Zealand.
"Staging the game at the Millennium Stadium will provide a unique opportunity for countless Lions fans to see the team on home soil and give the players an amazing send off to New Zealand."
John Feehan, CEO of the British & Irish Lions added: "We are delighted to be able to stage the Lions game against the Pumas in Cardiff and provide a rugby showcase.
"The Millennium Stadium put forward the most impressive tender in the pitch process to win the bid to stage this high profile game and I have no doubt that this is the perfect venue for the Lions to kick off their 2005 campaign."
David Moffett, Group Chief Executive, WRU said: "Hosting the Lions home test is fantastic coup for the Millennium Stadium and for Wales. We appreciate that it is a unique event in Lions history. The Lions have only played in Europe on five previous occasions, three of which were staged in Wales. We are delighted to welcome the Lions back to Wales for what we hope will be a classic encounter.
"However, we know it is the supporters who are the key element in making the Lions so special and we looking forward to welcoming fans from England, Ireland, Scotland and Wales to the Welsh capital. We know they will create a remarkable atmosphere for the game - one that will hopefully become the stuff of Lions' legend."
Mr Emilio Perasso, President of the UniÃ³n Argentina de Rugby, said: "It is a great honour for Argentina, a top 10 rugby nation, to be asked to play against the British & Irish Lions and to provide their first challenge before the team head out to New Zealand.
"We know that it will be an unforgettable experience for our team. Not only on the pitch through the rugby, but also off it as we know their supporters will create a passionate rugby atmosphere at the Millennium Stadium. I am convinced that this fixture will further strengthen the links between Argentinian and British & Irish rugby, whose rugby forefathers helped to build rugby in Argentina over 100 years ago."