Proximity pitched against three other unnamed agencies to scoop the task of handling Royal Mail's digital strategy, online advertising, e-marketing and the redesign of the website.
John Baker, Proximity London's head of digital services, said interactive marketing would play a large part in Royal Mail's current restructuring.
"Royal Mail is going through a business transformation and interactive marketing will play a large part in this process. We have a solid vision for both the redesign of RoyalMail.com and the integration of interactive marketing with all of its marketing activity."
Proximity and sister above-the-line network Abbott Mead Vickers BBDO already hold Royal Mail's £40m above- and below-the-line marketing accounts, which they won under the moniker The Omnicom Consortium last month.
Steve Sefton, head of interactive marketing at Royal Mail, said: "A lot of great work was presented in the review, but Proximity's team showed us an interactive strategy and creative execution that was clearly the strongest solution. The fact that they are part of the Omnicom team is an added benefit. We are looking forward to working together to make Royal Mail's use of interactive channels best in class."
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