The brand, which comes in three flavours designed to mix well with Smirnoff Vodka and other spirits in the Diageo stable, is an attempt by the manufacturer to boost sales in pubs following a rise in the supermarket alcohol trade.
The business, which was awarded without a pitch, is the agency's first assignment from the drinks company.
Karmarama has been briefed to develop a multi-media campaign to unveil the brand later this month.
It will include an outdoor element, consisting of branded vehicles. A 48-sheet poster drive will be supported by in-bar promotional work , aimed at raising awareness of the mixers, which will go on sale initially at selected bars in major UK cities.
Naresh Ramchandani, Karmarama's co-founder and joint creative partner, said: "23 is an excellent product and stands a great chance of success."
Media planning and buying for Diageo is handled by Carat.