It will offer 3,000 hours of content from Channel 4, Five and archived programmes from BBC Worldwide and independent producers. However, the company does not have an output deal with ITV, which has been more reluctant to agree to third-party content arrangements.
UK Viewers can watch TV shows on SeeSaw for free. But it may start charging for the premium US and BBC content, and add another 2,000 hours of programming.
The internet TV company has promised ad-free display and fewer ads within programmes. Revenue will come from two 60-second ad slots a show, which will be split between the content producer and SeeSaw.
The service has been launched after two weeks from the date of beta testing on 2,000 users.
SeeSaw, owned by broadcast transmission group Arqiva, is based on video-on-demand technology bought from a defunct attempt by broadcasters ITV, Channel 4 and BBC Worldwide to set up their own consumer video service, named Project Kangaroo.