The agency won the business after a pitch process, having previously been appointed to handle the advertising for Project Kangaroo, the proposed VoD service backed by BBC Worldwide, Channel 4 and ITV, which was blocked from launching by the Competition Commission.
SeeSaw uses the technology originally intended for the defunct Project Kangaroo after Arqiva acquired the right to use it in July.
Fallon will create a through-the-line advertising campaign for SeeSaw to drive awareness of the service to consumers.
Maya Bhose, the head of brand marketing at SeeSaw, said: "The marketing campaign will form a key element in driving awareness of the service and engaging customers with SeeSaw."
Last week, SeeSaw hired Vizeum to handle media planning and buying.