The Powergen Challenge Cup Final first came to Wales in 2003 and will return for the 2004 final on 15 May.
"We are delighted to welcome back the Rugby Football League for a third year. It seems the Millennium Stadium is becoming something of a home from home for the league fans," said Millennium Stadium manager Paul Sergeant.
"Events of this stature are not just good business for the Millennium Stadium, but fantastic for the Welsh economy as a whole. The Welsh sporting public have already embraced the Powergen Challenge Cup Final and we are expecting another 70,000+ crowd in May for the 2004 Final."
Richard Lewis, the Rugby Football League's Executive Chairman, said: "We're announcing the date for the 2005 Powergen Challenge Cup Final because next year the competition will occupy a different time frame within the RugbyLeague fixture calendar.
"We are conscious that Rugby League fans should have as much notice as possible to make their plans.
"And we want as many fans as possible to have the opportunity to join us on what should not only be an historic day for the sport and the tournament, but the start of an exciting spell for the game as it leads into the Super League Play off and Grand Final as well as the Tri-Nations tournament".