Based on technology developed for collapsed joint venture Project Kangaroo, the Arqiva-owned service provider today confirmed deals for around 3,000 hours of programming.
The platform, which entered closed beta on Tuesday, will host Channel 4's 4OD and Five's Five On Demand when it fully launches around March.
A deal is already in place with BBC Worldwide for a range of BBC programming, leaving just ITV remaining from the terrestrial networks.
However, SeeSaw has reached a separate agreement with UK independent distributor Digital Rights Group, which will result in flagship ITV1 dramas, such as Doc Martin and Kingdom, being available at launch.
The firm has also agreed the "first of many" US content deals with RHI Entertainment for 50 made-for-TV movies. All announced content will be available for free on the site, but supported by pre and mid-roll advertising.
SeeSaw chief executive Pierre-Jean Sebert described the programming deals as a "major step forward" for the platform ahead of its commercial launch.
"These strategic partnerships, along with the recent BBC Worldwide deal, demonstrate SeeSaw's ambition to offer the richest and most diverse range of programming of any online TV service in the UK. And there's more to come," he said.
A dedicated sports section will be also created on SeeSaw to host a range of content and programming. For the section, SeeSaw today confirmed a deal with Perform - which sprung to notoriety last year after streaming an England international football game exclusively online - for a raft of football programming around the 2010 World Cup in June.
"Sport will play a crucial role in broadening the appeal of SeeSaw to British viewers," said Sebert, who was previously managing director of British Eurosport.
"The deal with Perform is the first of many and will offer topical, engaging and inspiring programmes for sport lovers across the country."
SeeSaw platform controller John Keeling added: "The breadth and depth of the content announced today instantly establishes SeeSaw's position as a very significant player in the online TV marketplace.
"As a free, homegrown television platform, scheduled for British audiences and advertisers, we look forward to raising the curtain on SeeSaw in the coming weeks."
Five chairman and chief executive Dawn Airey recognised SeeSaw as the "newest addition to the television playground".
She also praised the platform as "another safe and legal way for people to watch their favourite shows whenever, wherever and however they want to".
SeeSaw has also agreed separate deals with independent production firms Shed Media and TalkbackThames for a variety of programming.
TalkbackThames commercial director Rupert Brankin-Frisby said: "We are delighted to participate in the SeeSaw launch. Making our content available on online platforms is a key part of our distribution strategy and will benefit content owners and viewers alike."