William, who worked as an RAF search rescue pilot in Anglesey until September, was given a tour of the Welsh team's changing room at Millennium Stadium by the club's Chief Executive Roger Lewis.
Wearing a fitting dark red tie, the 31-year-old Prince met injured former players who are beneficiaries of the Welsh Rugby Charitable Trust.
The trust, of which William is patron, was set up in 1972 to support players that have been seriously injured while playing rugby for Wales.
After his tour of the stadium new dad William then took his seat in the stands to watch Wales take on the South African Springboks.
Earlier in the week during a visit to St Giles Trust in London, the Duke of Cambridge said that he would have his "fingers crossed" for the Welsh side come Saturday.
This is not the first time William has turned out to support Wales at a rugby match. William and Kate cheered their adoptive home in November 2012.
The Duchess paid a sartorial tribute to the Welsh team, wearing a scarlet coat by L.K. Bennett which she recyled just two days ago for London Poppy Day.
Watching rugby elsewhere in the UK was Prince William's friend, footballing legend David Beckham.
David and his three sons Brooklyn, Romeo and Cruz were spotted at Twickenham watching England play Argentina.