Big Brother 2011, also known as Big Brother 12, was the twelfth series of the British reality television series Big Brother and the first not to be broadcast on Channel 4. It was broadcast on Channel 5 for the first time since the show’s transfer from Channel 4. It launched on 9 September 2011 with an hour and a half-long special launch show, the day after the final of Celebrity Big Brother 8. It was hosted by Brian Dowling, the winner of Big Brother 2 and Ultimate Big Brother. The series ran for 64 days, ending on 11 November 2011 when the winner, Aaron Allard-Morgan, won half of the £100,000 prize fund, with the remainder split between the five finalists. The runner up was Jay McKray.

The series differed from the Channel 4 version in having the celebrity edition before the main series, with the latter running in the autumn rather than the traditional summer-long run. The series reverted to its traditional pattern a year later when Big Brother 13 aired in the summer of 2012. In 2018 the series reverted to the 2011 format again with Celebrity Big Brother 22 airing before Big Brother 19.

There were 14 original housemates, seven men and seven women all aged between 18 and 30. Much of the series concentrated on the romantic relationships that developed between the housemates during their time in the house.

The series was sponsored by skincare brand Freederm and was the first to use online voting via Facebook credits.