The multi-channel campaign launched on Monday with national primetime TV ads, supported by radio, print and online activity, and marks the largest marketing push undertaken by a UK job site.
Jobsite, which previously used an in-house marketing team, handed the creative account to Hooper Galton.
Publicis Groupe's MediaVest was awarded media planning and buying for the campaign and communications firm Monkey is responsible for its communications strategy.
Nick Jones, I Spy director, said: "I Spy are delighted to have been chosen by Jobsite and to be part of such an extensive and innovative branding campaign.
"With our extensive knowledge of executing highly successful paid and natural search strategies in the recruitment sector, whilst integrating it with other media channels, we feel we really are the ideal partner for Jobsite and look forward to building a long-term relationship."