Ashton Kutcher and Natalie Portman star as friends who try to maintain a purely sexual relationship, even as they're falling in love.
Last week's top film, "The Green Hornet," dropped to second place with $18.1 million.
"The Dilemma" was in third place with $9.7 million.
In fourth place, "The King's Speech" earned $9.2 million and "True Grit" rounded out the top five with $8 million in earnings.
The top 10 movies of the weekend were:
1. "No Strings Attached," $20.3 million.
2. "The Green Hornet," $18.1 million.
3. "The Dilemma," $9.7 million.
4. "The King's Speech," $9.2 million.
5. "True Grit," $8 million.
6. "Black Swan," $6.2 million.
7. "The Fighter," $4.5 million.
8. "Little Fockers," $4.4 million.
9. "Yogi Bear," $4.1 million.
10. "Tron: Legacy," $3.7 million.
