The French-owned outdoor giant won the Transport for London bus shelter contract from Clear Channel last year.
This week's move is the start of what the company calls a "digital transformation" with 1,000 new screens going up in leading locations, including Covent Garden, Holborn and Knightsbridge.
The bus shelters will have 84-inch, connected, dynamic HD screens which were described by JCDecaux as "the largest ever deployed at scale", adding nearly 40 per cent to the screen size.
The new bus shelter screens will cover the capital’s major shopping areas, accounting for £1 in every £5 of the UK’s retail spend, according to data from JCDecaux.
The rollout comes just a week before TfL decides its £1 billion Tube advertising contract for which JCDecaux is bidding against Exterion Media, the incumbent.
Jean-François Decaux, the chairman and co-chief executive of JCDecaux, said: "The start of our digital bus shelter transformation doesn’t just mark an important milestone for JCDecaux, but also for London.
"We are committed to making London the global showcase for digital out-of-home and forecast more than 50 per cent of UK advertising revenues to be coming from digital by 2017."
JCDecaux admitted last week in its annual results that UK revenues rose only 0.4 per cent on an organic basis, blaming "softness" in the market last year.