The duchess said she will be "glued to the TV" ahead of Wales' Euro 2016 semi-final match against Portugal.
She spoke while visiting Cardiff Bay on the third day of the royal couple's annual Wales tour.
In Pontypridd, the royal couple was presented with Wales football kits for grandchildren Charlotte and George.
Asked by reporters about the football match, the duchess said: "We'll all be glued to the TV tonight, keeping our fingers crossed."
On Tuesday, the prince also backed the Wales side, wishing them "huge good luck".
The royal couple also visited Pontypridd's Lido Ponty, a Grade II listed outdoor pool which has been restored.
At the Norwegian Church in Cardiff Bay, the Duchess met children who had written songs based on Roald Dahl's work - part of the celebrations of the centenary of Dahl's birth.
Later, at the Wales Millennium Centre (WMC), she sampled a drink made from a machine inspired by Dahl's book George's Marvellous Medicine.
The machine will form part of the City of the Unexpected theatre event in Cardiff in September.
Prince Charles went to Brecon, where he met young people from his Prince's Trust charity, and visited the Central Beacons Mountain Rescue Team.
This year marks Charles's 12th annual summer visit to Wales - which sees him stay at his 192-acre Llwynywermod estate near Llandovery.
Prince Charles and the Duchess of Cornwall wore Football Association of Wales badges as they visited south Wales on Wednesday.