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 to make a single payment.
The agency has been appointed with immediate effect, ahead of a series of fundraising and high profile events this Thursday, to mark "wear it pink" day, the culmination of the charity's Breast Cancer Awareness Month.
Recent campaigns from the charity include a call for better screening for women over 70 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."