The Royal Channel will feature her Christmas Day message as well as recent and historical footage of the monarch and other members of the Royal Family.
The launch marks the 50th anniversary of the Queen's first televised festive address in 1957.
The palace said it hoped the site would make the 81-year-old monarch's annual speech "more accessible to younger people and those in other countries".
The opening page of the channel, which went live just after midnight, bears the title "The Royal Channel - The Official Channel of the British Monarchy" and features a photograph of Buckingham Palace and the Queen's Guards.
This year's festive address will appear on the site at about 1500 GMT on Christmas Day.
"She has always been aware of reaching more people and adapting the communication to suit." Buckingham Palace spokeswoman
Back in 1957, when the Queen delivered her first television message, she acknowledged the need to adapt to changing times.
"I very much hope that this new medium will make my Christmas message more personal and direct," she said from her Sandringham estate in Norfolk.
"That it is possible for some of you to see me today is just another example of the speed at which things are changing all around us."
Clips from garden parties, state visits, prime ministers, investitures and a day in the life of the Prince of Wales will all be available to watch on the channel.
Among the older clips is footage from a film by Lord Wakehurst called Long to Reign Over Us, which has never been released to the public.
The former Tory MP, who died in 1970, was a keen amateur film maker and charted many key royal events, including the death of King George VI, the Queen's accession and her coronation.
The site also has footage of Queen Alexandra's West End tour among the rose-sellers in 1917 and silent newsreel of the 1923 wedding of the Duke of York and Lady Elizabeth Bowes-Lyon - the Queen's parents.
Announcing the launch of the channel, a spokeswoman for Buckingham Palace said the Queen "always keeps abreast with new ways of communicating with people".
She added: "She has always been aware of reaching more people and adapting the communication to suit.
"This will make the Christmas message more accessible to younger people and those in other countries."
The Royal Channel can be viewed at www.youtube.com/theroyalchannel and the Queen's Christmas message can also be downloaded as a podcast from www.royal.gov.uk.