Bale, who helped guide Madrid to Copa del Rey and Champions League glory in his first season at the Bernabeu, will discuss the importance of making the right decisions in the television advert, which will air this weekend.
The ads, which will also feature in print format, will aim to encourage viewers to choose BT Sport for the new season as they prepare to show top-flight football from England, France, Germany and Italy.
BT consumer chief executive John Petter said: "BT Sport is the home of European football and we are delighted to work with Gareth to reinforce this message.
"He is one of the world's greatest players and having helped Real Madrid win the UEFA Champions League final this season, he knows what he is talking about."
BT Sport will show live coverage of numerous European club cup competitions from August 2015.