The Welsh forward will appear in an advertising campaign to promote the launch of BT Sport in the UK, as well as its live coverage of 38 live Premier League games a season from August 2013. He will also appear as a regular guest on BT Sport programmes.
The 23-year-old has an existing relationship with BT, having promoted its Life's a Pitch website and the availability of Sky Sports on the BT Vision pay-TV platform in 2010.
"I am delighted to continue my relationship with BT by becoming a BT ambassador," said Bale. "I feel honoured to join an exciting band of inspirational sporting figures, which has included Olympic heroes like the Brownlee brothers and Paralympic gold medallist Jonnie Peacock.
“A great deal has changed in my football career since my first partnership with BT, and I am happy to extend that relationship for the next three years. I am excited about the launch of BT Sport and I am happy to play my part in making it a big hit."
Simon Green, the head of BT Sport, added: "We are thrilled that Gareth has agreed to feature in our TV adverts and some BT Sport programmes, as well as becoming a BT ambassador.
“He has developed into one of the most-admired and coveted players in the Barclays Premier League in recent years. He is also an inspiring role model for young sports fans, who remains dedicated and focused on developing his considerable talent.
“We are delighted that we’ve managed to do what dozens of top football managers would like to do – sign Bale."