The agency will work across all markets other than the US, where the business remains with Blue Flame.
Cîroc has traditionally avoided advertising in the UK, preferring instead to spend its budget on music alliances.
Since 2007, Cîroc has had a profit-share deal with the musician and entrepreneur P Diddy, who acts as its "chief marketing officer and brand ambassador" and hosts special events that feature the brand. These are then promoted on social media.
A spokeswoman for Diageo said: "Diageo can confirm that, following a pitch process, it has appointed Hometown London to work on the Cîroc vodka brand in markets outside of the US."