The figure, which includes viewing on C4+1, accounted for 25% of the total viewing audience. It is a fall of 800,000 from 2007, when 6.2m (26%) watched the start of Big Brother 8.
The first episode of the seventh series attracted 6.9m (30.6%).
Channel 4 said last night's show attracted a strong 41% share of viewing among 16- 34-year-olds.
Later on, 2.1m (16.5%) watched The Big Brother Launch Night Project at 10.40pm, 382,100 (2.9%) watched E4's live stream from 10.30pm to 11.40pm, and 314,000 (4.9%) saw this year's first Big Brother's Big Mouth at 11.40pm.
Angela Jain, commissioning editor for Big Brother and Head of E4, said: "This is a strong start for Big Brother 9 and it's fantastic that over a quarter of everyone watching TV last night tuned in to see the launch.
"We remain as committed as ever to making this year's series the most entertaining yet."