Directgov, which receives 2.7m visitors a month, is the central portal for public information about all government services. It became part of the COI in April, having previously been managed by the Cabinet Office.
The appointment follows a pitch in May, when eight agencies were invited to review the site and develop branding ideas.
Bostock & Pollitt has been briefed to develop a more customer-focused brand for Directgov as it integrates more content from government departments.
Agencies appointed to the COI's digital roster in June will also be involved in the development of the portal.