The Big Brother final drew a peak audience of 7.8 million viewers who saw Anthony Hutton win the £50,000 prize.
The Channel 4 show drew an estimated average of 6.7 million viewers, beating the average 6.6 million who saw last month's Live 8 concerts on BBC One.
Channel 4's highest-rating show of 2005 so far, it still fell short of the 9.5 million who watched the 2000 final.
Anthony from County Durham beat Eugene Sully in one of the closest-fought votes in the reality show's six series.
Eugene, a former BBC engineer from Crawley, also claimed £50,000, after Big Brother offered half the £100,000 prize fund to him on Wednesday.
Kinga Karolczak and Makosi Musambasi were earlier evicted from the Big Brother house as the series reached its climax, with Kinga in fourth place and Makosi in third.
At its peak Friday night's final was watched by an estimated 43.4% of the total available TV audience.
Channel 4 commissioning editor Angela Jain said: "We are thrilled that Big Brother has once again been such a fantastic success.
"It is a show that consistently reinvents itself. We look forward to seeing it back on our screens next summer."
The channel has signed an agreement with Big Brother producers Endemol for two further series of the reality show.
On leaving the Big Brother house on Friday, 1970s dancer Anthony said: "I haven't got the vocabulary to describe how I feel but I feel absolutely mint."
Eugene, 27, described Anthony as a "great person" and a worthy winner.
"I was very chuffed to still be in the final, let alone come second," he said.
Earlier this week Eugene became £50,000 richer after being offered a choice to take half the prize money or leave it all for the eventual winner.
Anthony said he was not disappointed that the top prize fund had depleted thanks to Eugene's decision.
"If it had been a toothbrush I had won I would have wanted to do it," Anthony said.
"If I had a choice of £100,000 or winning I would have picked winning any time."