Propeller did not competitively pitch for the account. JCDecaux had previously run its comms in-house.
The ad firm recently won the TfL bus shelter account and is launching Dynamic, a digital creative hub, to boost its digital offering.
Propeller has been briefed to help raise awareness of the company and its data and digital offering.
JCDecaux marketing director David McEvoy said: "Having won the London bus shelter contract in 2015, 2016 will be a landmark year for JCDecaux. Already one of the UK’s most successful media owners, we are investing heavily to digitise our network to push us further ahead of our rivals and transform out-of-home advertising.
"With digital, and the smart use of our data and technology, we give advertisers the opportunity to create highly targeted, highly imaginative outdoor campaigns. Propeller's deep media industry connections and reputation for results makes the agency the perfect partner to help us tell our story."
Martin Loat, chief executive of Propeller PR, added: "JCDecaux is in the ascendancy and has a long-term vision. Embracing digital to its full potential is the next step on the company’s journey."