The sci-fi movie, produced by the Lord of the Rings filmmaker and directed by 29-year-old Neill Blomkamp, is the surprise hit of the summer - trumping the hotly-rated The Time Traveler's Wife.
The Eric Bana and Rachel McAdams-starring romance film came in third with $19,205 million (£12.8 million), behind.
G.I. Joe: Rise of the Cobra, which took $22,500,000 (£15 million.
Julie & Julia and G-Force wrap up the top five, while Jeremy Piven comedy The Goods: Live Hard, Sell Hard had to settle for sixth place, taking just $5,350,000 (£3.56 million).