The media business was previously split between Rocket, which handled planning and buying; Kinase, which oversaw pay-per-click and biddable activity; and the SEO agency Mode Digital.
ZenithOptimedia is believed to have beaten all the incumbent agencies (bar Mode Digital), as well as MEC, M2M and Forward3D.
Shadi Halliwell, the group marketing and creative director at Harvey Nichols, said: "Harvey Nichols is embarking on an exciting journey and we require a full-service media partnership that can challenge us and support the delivery of our ambitious growth plans.
"ZenithOptimedia’s credentials in the luxury market shone out among the competition. They demonstrated not only new innovation but a deep understanding of the brand and how to effectively reach our target audience."
Adam & Eve/DDB is Harvey Nichols’ creative agency.