Trailing not too far behind was another action film. "I Am Number Four" opened at No. 2 with $19.5 million.
In 'Unknown,' Neeson stars as a botanist who wakes up from a coma following a car crash and finds his wife claims she does not know him and that another man has taken over his identity.
While Neeson's film drew in a more mature crowd, "I Am Number Four" held youth appeal. The DreamWorks film stars Alex Pettyfer as a teenage alien on Earth with superpowers.
Two movies that opened the previous weekend held the next two spots in the top 10.
Disney's animated comedy, "Gnomeo & Juliet," was No. 3 with $19.4 million, and Adam Sandler and Jennifer Aniston's romance "Just Go With It" came in at No. 4.
The weekend's other new wide release, Martin Lawrence's comedy, "Big Mommas: Like Father, Like Son," round out the top 5 with $17 million.
1. "Unknown," $21.8 million.
2. "I Am Number Four," $19.5 million.
3. "Gnomeo & Juliet," $19.4 million.
4. "Just Go With It," $18.2 million.
5. "Big Mommas: Like Father, Like Son," $17 million.
6. "Justin Bieber: Never Say Never," $13.6 million.
7. "The King's Speech," $6.6 million.
8. "The Roommate," $4.1 million.
9. "The Eagle," $3.6 million.
10. "No Strings Attached," $3.1 million.
The Associated Press contributed to this report.
