The 24-year-old singer - real name Stefani Germanotta - climbs to the top spot with her debut single, 'Just Dance'.
Rock falls to second with 'All Summer Long', while Pink holds steady in third with 'So What'.
Meanwhile, Katy Perry climbs from 16 to seven with 'Hot N Cold', the follow-up to her international chart-topper 'I Kissed A Girl'.
The top ten singles in full (click where possible for our reviews):
1. (2) Lady GaGa: 'Just Dance'
2. (1) Kid Rock: 'All Summer Long'
3. (3) Pink: 'So What'
4. (4) Metro Station: 'Shake It'
5. (5) Jason Mraz: 'I'm Yours'
6. (7) Rihanna: 'Disturbia'
7. (16) Katy Perry: 'Hot N Cold'
8. (6) Pussycat Dolls: 'When I Grow Up'
9. (8) Katy Perry: 'I Kissed A Girl'
10. (9) The Veronicas: 'Take Me On The Floor'