The agency was appointed without a pitch and will handle launch advertising for the website, which attempts to offer a pain-free conveyancing service for house-buyers.
M&C Saatchi will be responsible for a campaign later this year that will include TV, print and digital elements.
In-deed is the brainchild of Harry Hill, the founder of the property website Rightmove, and Peter Gordon, a former executive at the private equity company 3i.
Richard Alford, the managing director of M&C Saatchi, said the agency will work with In-deed to make it a household name.
Gordon, the managing director of In-deed, said: "Conveyancing is a billion-pound industry that needs a shake-up. Together with the creative brains at M&C Saatchi, we plan to do just that."