Shows such as The Gadget Show, Neighbours and Home And Away are now available without charge from YouTube's new Shows section.
However, rights deals are still being negotiated to bring US programmes such as FlashForward and CSI to the site.
Demand Five catch-up content will reach YouTube shortly after TV transmission and users will be able to browse 250 hours of the broadcaster's archive content. Five intends to generate revenue from the section through advertising sales.
Five chairman and chief executive Dawn Airey said: "This is a tremendously important deal for Five because in one fell swoop it extends the reach of our content beyond linear TV and our own existing websites to a new audience of younger, upwardly mobile and web-savvy individuals - an audience that advertisers are equally desirous of attracting.
"Legitimate internet video operators are far from the 'parasites' that some have sought to portray them as in the past; on the contrary, they can be an important partner in helping modern day broadcasters reach new audiences and continue to fund high-quality original content."
In October, Channel 4 also agreed a deal with YouTube to make its 4oD catch-up service available for free on the streaming site.