This is the first time that the hospital has used an agency. Its previous fundraising efforts were produced in-house. HTW was recommended by Agency Insight and will be providing its services free of charge.
The decision to use a direct marketing agency follows the hospital's appointment of a new appeals director, Angela Roden. Roden previously worked for the Manchester cancer hospital Christies.
Roden said: "Our problem is that everyone has heard of the hospital but not everyone is aware of what we do."
HTW will carry out targeted direct marketing campaigns for the hospital over the next two to three years, and will be responsible for producing its promotional literature.
Steve Harrison, the creative director at HTW, said: "While we have been offering voluntary work for such causes as the Prince's Trust and the Starlight Children's Foundation, we've spent a long time looking for the right charity with whom we can have a long-term relationship. The Royal Marsden is perfect."