Frank PR confirmed it had been awarded the PR account for the company, which is also called Weetabix, but declined to comment further.
Good Relations and Mischief have previously handled consumer PR for the company. The former's hold on corporate and trade PR and press office management is unaffected by the change.
Cake won the social media brief after a four-way pitch overseen by Oystercatchers, conducted separately from the PR account.
The brief includes managing the social media channels for Weetabix and Alpen, as well as product launches.
Cake will report to Weetabix global marketing director Sally Abbott, who said: "Cake’s approach to creativity in the digital space, coupled with rigour in online reporting and process, really impressed us.
"We were looking for an agency that would position us firmly in the lead when it comes to digital and that would be ready to get their hands dirty. This made Cake the obvious choice for us."
The company’s other marketing agencies include BBH for advertising, Walker Media for media planning and buying and The Marketing Store for sales promotion.