The split has been attributed to creative differences less than a year after the coffee chain's first TV campaign.
The campaign, which broke late last year in the US, introduced the line 'Pass the cheer'. One of the animated spots in the campaign showed penguins handing a cup of coffee to a window washer.
Starbucks and Wieden + Kennedy have released a joint statement in which Terry Davenport, the senior vice-president of marketing at Starbucks, said the company had recently "made the decision to evaluate our agency work in support of the overall brand.
"We are asking a number of our current agencies to provide ideas to move the brand forward and as a result, Wieden + Kennedy has decided to opt out of the process."
Dan Wieden, chief executive of Wieden + Kennedy, said: "There are times when it just makes sense to part ways with a client. In this case, this seems to be the best decision for both parties."
The move comes after Starbucks announced in July it was to axe 600 stores in the US on the back of falling sales.
The closures, which will be implemented by the middle of next year, are designed to boost Starbucks' long-term profits.
Of the closures, 70% of the stores affected were opened after the beginning of the 2006 fiscal year.
Starbucks spent $55m (£29.6m) on ads in 2007 and almost $25m (£13.5m) through June 2008, according to Nielsen.
Wieden + Kennedy won the business, which includes both creative and media duties, in April 2004 following a competitive pitch process.