The 24 Havas offices in London, including Havas Worldwide London, AIS London and Arena, will start moving into the 11-storey, 165,000-sq ft building, which is on the northeast corner of Pancras Square, in 2017.
Under the plans, the ground floor of the building will have a double-height reception, accessed from King's Boulevard and direct into a café area from Pancras Square. There will be private roof terraces on floors 7, 8 and 9.
The decision to move all the London agencies into a single building follows a similar move in Paris and New York.
Yannick Bolloré, the chief executive of Havas, said "I'm very excited about our future headquarters in King's Cross. It is a further step in the implementation of our Havas Village strategy.
"The convergence of all our London-based agencies and their various disciplines is our guarantee to our clients that we can look beyond our individual expertise and work together to provide them with clear benefits in a constantly evolving market.
"Another factor in choosing King's Cross was its proximity to prestigious art and tech institutions."
The commercial property specialists JLL advised Havas with its search for a building. Savills advised the developers of the site, The King's Cross Central Limited Partnership.