Big Brother 2004, also known as Big Brother 5, was the fifth series of the British reality television series Big Brother. The show followed thirteen contestants, known as housemates, who were isolated from the outside world for an extended period of time in a custom built House. Each week, one or more of the housemates were eliminated from the competition and left the House. The last remaining housemate, Nadia Almada, was declared the winner, winning a cash prize of £63,500.

The series launched on Channel 4 on 28 May 2004 and ended on 6 August 2004. It lasted 71 days – the longest series of Big Brother at the time and the eighth longest series to date. Davina McCall returned as presenter for her fifth consecutive year. Twelve housemates entered on launch night, with one additional housemate being introduced in the fifth week.

Following the negative reception of the “boring” previous series, several changes were made to the format. Big Brother became “evil”, resulting in a higher-pressure environment for the housemates, more difficult tasks, fewer luxuries and more twists. Big Brother 5 ultimately became most memorable, and the subject of viewer complaints and press attention, for a physical altercation between housemates in the early hours of Day 20, which later became referred to by fans and in the press as “Fight Night”.

The series was watched by an average of 5.1 million viewers, the second highest viewed series of the show to date (after Big Brother 3).