Jenkins, who recently signed a multi-album deal with recording giants Universal Classics will eventually record the song for a special Rugby World Cup album. But Wales's star rugby players and the Millennium Stadium VIPs will be the first to hear her sing the new version live.
"I'm over the moon to be given an opportunity like this," said Jenkins. "I couldn't be more thrilled, it's something I have dreamed of since childhood - singing for Wales.
"And singing for the Welsh team on Wednesday as a message of farewell before they fly off for the Rugby World Cup will be a great honour."
The WRU barbeque will also feature a fashion show with Wales players and models showing off their new Rockport gear and a charity auction for various items of rugby and football memorabilia will raise money.
The food for the BBQ will include Celtic Pride Premium Welsh Beef produced under an innovative protocol designed to make it the finest in Wales, by the Welsh meat company.
"Kathryn is a very talented singer and we are extremely pleased to have secured her services to sing for the team," said WRU commercial manager Gwyn Thomas.
"Those who came to the recent Wales versus England game will remember her fondly.
"We are all looking forward to Wednesday and it will be great to see the different aspects of Welsh society working together to give the guys a great send off."