SeeSaw, which will launch in the next few months, was born out of the remnants of Project Kangaroo, which was blocked from launch by the Competition Commission earlier this year.
ITV, Channel 4 and BBC Worldwide hoped Kangaroo would rival the likes of YouTube.
Vizeum won the account following a pitch against the agencies that were involved in the original £4m Project Kangaroo pitch, which are understood to have included Mindshare, MPG and OMD.
The deal was concluded by Maya Bhose, head of brand marketing at SeeSaw, and Stuart Newman, managing partner, planning at Vizeum. The agency will start work immediately.
Bhose said its key objective was to appoint an agency with a "thorough grasp of the fast-evolving multi-platform landscape".
SeeSaw will offer popular British content supported by international and US programming. It is understood to have signed Channel 4 and BBC Worldwide as content partners, with more expected to be announced in the forthcoming weeks.
The search for an ad agency for SeeSaw is ongoing.
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