The film by Pixar features the voices of Ian Holm and Peter O'Toole and took $47.2 million in its first weekend.
Last week's number one Evan Almighty dropped down into third place.
The US box office top ten in full:
1. Ratatouille - $47.2 m
2. Live Free or Die Hard - $33.1m
3. Evan Almighty - $15.1m
4. 1408 - $10.6m
5. Fantastic Four: Rise of The Silver Surfer - $9m
6. Knocked Up - $7.42m
7. Ocean's Thirteen - $6.05m
8. Pirates of the Caribbean: At World's End - $5.01m
9. Sicko - $4.5m
10. Evening - $3.51m