The move ends Grey’s 14-year tenure on the business and follows a pitch process.
Aquafresh’s global advertising, including a UK spend of £10 million, will now be led out of TBWA\London.
The appointment will strengthen the agency’s existing relationship with GSK, which involves advertising for brands including Alli and NiQuitin.
Grey will continue to work on GSK’s healthcare range and oral hygiene brands including Sensodyne. It also handles advertising for the Lucozade energy drink.
Grey’s recent work for Aquafresh included a repositioning of the toothpaste brand after launching a campaign for its fruit-flavoured variant, which features the strapline: "Experience a new zone."
Last year, Aquafresh moved into broader content creation with the launch of a ten-minute film by the creative agency Kids Industries as part of a 12-month sponsorship deal with Cineworld Cinemas to promote its children’s toothpaste.
A GSK spokeswoman said: "TBWA was chosen as it demonstrated tremendous insight and creativity, which we believe will be invaluable as we look to reinvent consumers’ perceptions of Aquafresh."