Big Brother 2009, also known as Big Brother 10, was the tenth series of the British reality television series Big Brother. The show followed a total of twenty-two contestants, 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 evicted by a public vote. The last remaining housemate, Sophie Reade, was declared the winner, winning a cash prize of £71,320.
The series launched on Channel 4 on 4 June 2009 and ended on 4 September 2009, lasting 93 days – the joint-second longest British edition of Big Brother to date (together with the seventh and ninth series, and one day shorter than the eighth series). Davina McCall returned as presenter for her tenth consecutive year. Sixteen housemates entered on launch night, with an additional six being introduced in later weeks.
Big Brother 10 was watched by an average of 2.5 million viewers. It was the lowest rated series of the show since its inception, becoming the first series to draw an average of less than 3 million, and remains the lowest rated series of Big Brother to be broadcast on Channel 4. During the series, Channel 4 announced that it would not be renewing its contract to show the series with Endemol once it expired in 2010, meaning that the subsequent eleventh series would be the last to be broadcast on Channel 4.