The Set includes Hotel Café Royal in London, The Conservatorium in Amsterdam and The Lutetia, which is due to open in Paris in 2017. The three hotels have been designed by architects such as Piero Lissoni, David Chipperfield and Jean-Michel Wilmotte.
Sauce Communications' brief will be to promote the hotels as being "of the moment", while also defining their times, locations and status as historical landmarks.
The Set has worked with Scott & Co in the past.
Brian Gore, director of brand & marketing for The Set, said: "As we work towards the opening of The Lutetia in Paris, The Set, as a collection of hotels created to redefine the concept of luxury travel, is entering an exciting new phase and can only benefit from the support Sauce Communications will provide."
Nicola Hancock, co-founder of Sauce Communications, said: "This is a fantastic account for Sauce as we grow our travel and lifestyle departments, and is a great win for our team."