He will also work alongside other senior creatives, including the incoming executive creative director Mick Mahoney, who joins in August.
Before joining Publicis in 2010, Pennycuick was digital creative director at DraftFCB and he is a previous creative director at LBi.
Throughout his career, Pennycuick has worked on brands including British Airways, Nike, Coca Cola, Sony and Google.
Ben Kay, the chief executive at RKCR/Y&R, said: "It’s fantastic to have Alix on board at RKCR/Y&R. He’s a great talent who will take a key role in keeping the agency at the leading edge of the communications industry."