CST is understood to have fought off competition from M&C Saatchi, The Red Brick Road and one other undisclosed agency to win the business. The pitch was handled by the AAR.
Go3's omega 3 fish oil supplements, owned by Findus, will be sold in Tesco and are aimed at busy families who find it difficult to eat the recommended two servings of fatty fish a week.
Beckham will feature on the packaging and in the ads to help motivate families to be more active and lead healthier lifestyles.
CST's launch ads will run in early 2009.
Go3 launched in the US last year, in partnership with David Beckham who said: "Playing soccer since I was a young boy has taught me that good nutrition and a balanced diet is important to remaining healthy and fit.
"I'm teaming up with GO3 to let adults and children know there's an easy and simple way to include Omega-3 oils into their daily life."
Go3 appointed direct and digital agency Lida to create its global website earlier this year and the brand's packaging and logo were designed by Smith & Milton.