The merged company will now be called Plus 1 Communications, and describes itself in a press release as "a fully integrated communications agency offering both PR and digital marketing solutions".
Plus 1 is also launching an office in Dubai. This will be led by AKH founder Anna Hope, who will also oversee the London PR operation.
Karen Lane, previously managing director of Plus 1 Digital, becomes group MD of the new firm and remains London-based.
Financial terms of the deal were not disclosed. The new group has a total of 35 staff globally across its three offices.
AKH's clients include the annual Winter Wonderland event in London's Hyde Park, and it has also worked with international circus group Cirque du Soleil. Plus 1 Digital clients include Etihad Airways and Barclays' online lending marketplace RainFin.
Lane said: "Plus 1 Digital has grown incredibly quickly over the past 18 months, working with some amazing global brands. To accelerate our growth we’ve realised the need for both an in-house PR team and a big presence in the Middle East, while continuing to build the core digital marketing business in the UK."