The one-minute film includes fresh images of how Madeleine might look now, including one of her with dark brown hair and tanned skin in case she has been living in north Africa.
The appeal, launched by the Child Exploitation and Online Protection Centre (Ceop), is targeted at a friend or relative of the person responsible for the little girl's disappearance.
Ceop Chief executive Jim Gamble said the clip's message, which has been translated into several languages, had been created with the help of psychologists to persuade the witness to "do the right thing".
He said: "The person we are looking to reach is likely to be a partner, family member, friend or colleague of the person or people who were involved in Madeleine's disappearance."
Madeleine was nearly four when she went missing from her family's holiday apartment in Praia da Luz in Portugal on 3 May, 2007 while her parents dined with friends nearby.
Ceop hopes people will spread the new film across the globe using blogs, e-mail and social networking sites such as Facebook and Twitter.