Big Brother 2004, also known as Big Brother 5, was the fifth series of the British reality television series Big Brother, which premiered on Channel 4 on 28 May 2004 and lasted 71 days until the live finale on 6 August 2004.

Following the poor reception and ratings for Big Brother 4, producers decided to cast a more controversial set of housemates and introduced an 'evil' Big Brother. This saw an increase in ratings when compared to the previous edition. In total, the series averaged 5.1 million viewers, thus making it the second highest rated so far, only behind Big Brother 3.

The premise of the series remained largely unchanged from the previous instalment in the series, in which a certain number of strangers lived in a house together with no communication with the outside world, filmed constantly during their time in the house, trying to stay in the house for as long as possible in hopes of being voted the winner by the viewing public. It was the first series to feature a prize fund of up to £100,000. Certain elements of the show which had been there present since Big Brother's inception in 2000 were removed, such as the hot water only being present for one hour in the morning, the vegetable patch and the chickens.