He is currently focusing on his military career and is a Second Lieutenant in the Household Cavalry (Blues and Royals) but was determined to be present at Wales's crucial Pool B match.
Roger Lewis, WRU Group Chief Executive said: "The Prince is a passionate rugby supporter and we are proud to have him as our Vice Royal Patron.
"As soon as the Rugby World Cup fixtures were announced, we learnt of William's desire to attend this match.
"We are looking forward to welcoming him to the Millennium Stadium because we know how much he particularly enjoys watching Wales play.
"He has met most of the squad and gets on well with them. The Australian match will be a big game for Wales and I know Prince William will look forward to experiencing a fantastic Rugby World Cup atmosphere at the Millennium Stadium a week on Saturday.
"His involvement with Wales and Welsh rugby is growing and we are delighted he makes the effort to come and visit us whenever he can."
Prince William wished the team well as they begin their World Cup preparations in France today, commenting, "I know the team will be very excited at the prospect of participating in one of the world's greatest sporting events.
"It's a once in a lifetime experience for most players and I wish the players all the very best for their opening game against Canada on Sunday and for the rest of the tournament."