Harvest will manage keyword creation, optimisation and bidding on the main search engines, such as Google and Yahoo!.
The campaign clicks through to a landing page, created in-house, on www.breakthrough.org.uk, featuring calls to action that users can click to sign up for monthly donations or make a single payment.
The agency has been appointed with immediate effect, ahead of a series of high profile and fund-raising events on Thursday this week called ‘wear it pink', the culmination of the charity's breast cancer awareness month.
Recent campaigns from the charity include a call for better screening for women over seventy, and a breast awareness campaign called ‘TLC: Touch, Look, Check'.
Emma Wilson, director at Harvest Digital, said: "Breakthrough Breast Cancer is an established charity brand but has yet to fully exploit online marketing. We are working with them to prove the channel as a new donor recruitment method, starting with search marketing."